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29 September 2009

Operating Systems concepts: Linux Kernel in depth


Computer software can be divided roughly into two kinds:
  • system programs: which manage the operation of the computer itself, and
  • application programs: which perform the actual work the user wants.
The most fundamental system program is the operating system, whose job is to control all the computer's resources and provide a base upon which the application programs can be written. A modern computer system is a a complex system. Writing programs that keep track of all its components and use them correctly, let alone optimally, is an extremely difficult job. If every programmer had to be concerned with how disk drives work, and with all the dozens of things that could go wrong when reading a disk block, it is unlikely that many programs could be written at all. Many years ago it became abundantly clear that some way had to be found to shield programmers from the complexity of the hardware. The way that has evolved gradually is to put a layer of software (to manage all parts of the system) on top of the bare hardware and present the user with an interface or virtual machine that is easier to understand and program. This layer of software is the operating system.

A computer system consists of (see Fig. above):
  • hardware
  • system programs and
  • application programs
hardware layer In many cases, is itself composed of two or more levels (or layers) where:
  • the lowest level contains physical devices, consisting of integrated circuit chips, wires, power supplies, cathode ray tubes, and similar physical devices. How these are constructed and how they work is the province of the electronic-electrical engineer.
  • Next comes the microarchitecture level, in which the physical devices are grouped together to form functional units. Typically this level contains some registers internal to the CPU (Central Processing Unit) and a data path containing an arithmetic logic unit(ALU). In each clock cycle, one or two operands are fetched from the registers and combined in the arithmetic logic unit (for example, by addition or Boolean AND). The result is stored in one or more registers. On some machines, the operation of the data path is controlled by software, called the microprogram. On other machines, it is controlled directly by hardware circuits.
  • The purpose of the data path is to execute some set of instructions. Some of these can be carried out in one data path cycle; others may require multiple data path cycles. These instructions may use registers or other hardware facilities. Together, the hardware and instructions visible to an assembly language programmer form the ISA (Instruction Set Architecture) This level is often called machine language. The machine language typically has between 50 and 300 instructions, mostly for moving data around the machine, doing arithmetic, and comparing values. In this level, the input/output devices are controlled by loading values into special device registers. Furthermore, for many I/O (Input/Output) devices, timing plays an important role in the programming.
OS layer A major function of the operating system is to hide all this complexity and give the programmer a more convenient set of instructions to work with. For example, read block from file is conceptually much simpler than having to worry about the details of moving disk heads, waiting for them to settle down, and so on.

System utilities layer On top of the operating system is the rest of the system software(sometimes also called userland utilities ). Here we find:
It is important to realize that these programs are definitely not part of the operating system, even though they are typically supplied preinstalled by the computer manufacturer, or in a package with the operating system if it is installed after purchase. This is a crucial, but subtle, point. The operating system is (usually) that portion of the software that runs in kernel mode
said also supervisor mode).
It is protected from user tampering by the hardware (ignoring for the moment some older or low-end microprocessors that do not have hardware protection at all). Compilers and editors run in user mode. If a user does not like a particular compiler, he's free to write his own if so chooses; he is not free to write his own clock interrupt handler, which is part of the operating system and is normally protected by hardware against attempts by users to modify it.
This distinction, however, is sometimes blurred in embedded systems (which may not have kernel mode) or interpreted systems (such as Java-based systems that use interpretation, not hardware, to separate the components). Still, for traditional computers, the operating system is what runs in kernel mode.
That said, in many systems there are programs that run in user mode but which help the operating system or perform privileged functions. For example, there is often a program that allows users to change their passwords. This program is not part of the operating system and does not run in kernel mode, but it clearly carries out a sensitive function and has to be protected in a special way.
In some systems, this idea is carried to an extreme form, and pieces of what is traditionally considered to be the operating system (such as the file system) run in user space. In such systems, it is difficult to draw a clear boundary. Everything running in kernel mode is clearly part of the operating system, but some programs running outside it are arguably also part of it, or at least closely associated with it. For example, in MINIX 3, the file system is simply a big C program running in user-mode.

application programs Lies above the system programs. These programs are purchased (or written by) the users to solve their particular problems, such as word processing, spreadsheets, engineering calculations, or storing information in a database.

What Is an Operating System? It is difficult to pin down precisely what an operating system is. Operating systems perform two basically unrelated functions and depending on who is doing the talking, you hear mostly about one function or the other.:
  • extending the machine: As mentioned earlier, the architecture (instruction set, memory organization, I/O, and bus structure) of most computers at the machine language level is primitive and awkward to program, especially for input/output. Let us briefly look at how floppy disk I/O is done using the NEC PD765 compatible controller chips used on many Intel-based personal computers. PD765 has 16 commands, each specified by loading between 1 and 9 bytes into a device register. These commands are for reading and writing data, moving the disk arm, and formatting tracks, as well as initializing, sensing, resetting, and recalibrating the controller and the drives.
    The most basic commands are read and write, each of which requires 13 parameters, packed into 9 bytes. These parameters specify such items as the address of the disk block to be read, the number of sectors per track, the recording mode used on the physical medium, the intersector gap spacing, and what to do with a deleted-data-address-mark. If you do not understand this mumbo jumbo, do not worry; that is precisely the point it is rather esoteric. When the operation is completed, the controller chip returns 23 status and error fields packed into 7 bytes. As if this were not enough, the floppy disk programmer must also be constantly aware of whether the motor is on or off. If the motor is off, it must be turned on (with a long startup delay) before data can be read or written. The motor cannot be left on too long, however, or the floppy disk will wear out. The programmer is thus forced to deal with the trade-off between long startup delays versus wearing out floppy disks (and losing the data on them).
    Without going into the real details, it should be clear that the average programmer probably does not want to get too intimately involved with the programming of floppy disks (or hard disks, which are just as complex and quite different). Instead, what the programmer wants is a simple, high-level abstraction to deal with. In the case of disks, a typical abstraction would be that the disk contains a collection of named files. Each file can be opened for reading or writing, then read or written, and finally closed. Details such as whether or not recording should use modified frequency modulation and what the current state of the motor is should not appear in the abstraction presented to the user. The program that hides the truth about the hardware from the programmer and presents a nice, simple view of named files that can be read and written is, of course, the operating system. Just as the operating system shields the programmer from the disk hardware and presents a simple file-oriented interface, it also conceals a lot of unpleasant business concerning interrupts, timers, memory management, and other low-level features. In each case, the abstraction offered by the operating system is simpler and easier to use than that offered by the underlying hardware.
    In this view, the function of the operating system is to present the user with the equivalent of an extended machine or virtual machine that is easier to program than the underlying hardware. How the operating system achieves this goal is a long story, which we will study in detail throughout this post. To summarize it in a nutshell, the operating system provides a variety of services that programs can obtain using special instructions called system calls.
  • managing resources The concept of the operating system as primarily providing its users with a convenient interface is a top-down view. An alternative, bottom-up view holds that the operating system is there to manage all the pieces of a complex system. Modern computers consist of processors, memories, timers, disks, mice, network interfaces, printers, and a wide variety of other devices. In the alternative view, the job of the operating system is to provide for an orderly and controlled allocation of the processors, memories, and I/O devices among the various programs competing for them.
    Imagine what would happen if three programs running on some computer all tried to print their output simultaneously on the same printer. The first few lines of printout might be from program 1, the next few from program 2, then some from program 3, and so forth. The result would be chaos. The operating system can bring order to the potential chaos by buffering all the output destined for the printer on the disk. When one program is finished, the operating system can then copy its output from the disk file where it has been stored to the printer, while at the same time the other program can continue generating more output, oblivious to the fact that the output is not really going to the printer (yet).
    When a computer (or network) has multiple users, the need for managing and protecting the memory, I/O devices, and other resources is even greater, since the users might otherwise interfere with one another. In addition, users often need to share not only hardware, but information (files, databases, etc.) as well. In short, this view of the operating system holds that its primary task is to keep track of who is using which resource, to grant resource requests, to account for usage, and to mediate conflicting requests from different programs and users. Resource management includes multiplexing (sharing) resources in two ways: in time and in space. When a resource is time multiplexed, different programs or users take turns using it. First one of them gets to use the resource, then another, and so on. For example(use of time multiplexing), with only one CPU and multiple programs that want to run on it, the operating system first allocates the CPU to one program, then after it has run long enough, another one gets to use the CPU, then another, and then eventually the first one again. Determining how the resource is time multiplexed who goes next and for how long is the task of the operating system. Another example of time multiplexing is sharing the printer. When multiple print jobs are queued up for printing on a single printer, a decision has to be made about which one is to be printed next.
    The other kind of multiplexing is space multiplexing. Instead of the customers taking turns, each one gets part of the resource. For example, main memory is normally divided up among several running programs, so each one can be resident at the same time (for example, in order to take turns using the CPU). Assuming there is enough memory to hold multiple programs, it is more efficient to hold several programs in memory at once rather than give one of them all of it, especially if it only needs a small fraction of the total. Of course, this raises issues of fairness, protection, and so on, and it is up to the operating system to solve them. Another resource that is space multiplexed is the (hard) disk. In many systems a single disk can hold files from many users at the same time. Allocating disk space and keeping track of who is using which disk blocks is a typical operating system resource management task.

History of Operating Systems
Operating systems have been evolving through the years. Since operating systems have historically been closely tied to the architecture of the computers on which they run, we will look at successive generations of computers to see what their operating systems were like. This mapping of operating system generations to computer generations is crude, but it does provide some structure where there would otherwise be none.
The first true digital computer was designed by the English mathematician Charles Babbage (1792-1871). Although Babbage spent most of his life and fortune trying to build his "analytical engine," he never got it working properly because it was purely mechanical, and the technology of his day could not produce the required wheels, gears, and cogs to the high precision that he needed. Needless to say, the analytical engine did not have an operating system.
As an interesting historical aside, Babbage realized that he would need software for his analytical engine, so he hired a young woman named Ada Lovelace, who was the daughter of the famed British poet Lord Byron, as the world's first programmer. The programming language Ada® was named after her.

The First Generation (1945-55) Vacuum Tubes and Plugboards
After Babbage's unsuccessful efforts, little progress was made in constructing digital computers until World War II. Around the mid-1940s, Howard Aiken at Harvard University, John von Neumann at the Institute for Advanced Study in Princeton, J. Presper Eckert and John Mauchley at the University of Pennsylvania, and Konrad Zuse in Germany, among others, all succeeded in building calculating engines. The first ones used mechanical relays but were very slow, with cycle times measured in seconds. Relays were later replaced by vacuum tubes. These machines were enormous, filling up entire rooms with tens of thousands of vacuum tubes, but they were still millions of times slower than even the cheapest personal computers available today.
In these early days, a single group of people designed, built, programmed, operated, and maintained each machine. All programming was done in absolute machine language, often by wiring up plugboards to control the machine's basic functions. Programming languages were unknown (even assembly language was unknown). Operating systems were unheard of. The usual mode of operation was for the programmer to sign up for a block of time on the signup sheet on the wall, then come down to the machine room, insert his or her plugboard into the computer, and spend the next few hours hoping that none of the 20,000 or so vacuum tubes would burn out during the run. Virtually all the problems were straightforward numerical calculations, such as grinding out tables of sines, cosines, and logarithms.
By the early 1950s, the routine had improved somewhat with the introduction of punched cards. It was now possible to write programs on cards and read them in instead of using plugboards; otherwise, the procedure was the same.

The Second Generation (1955-65) Transistors and Batch Systems
The introduction of the transistor in the mid-1950s changed the picture radically. Computers became reliable enough that they could be manufactured and sold to paying customers with the expectation that they would continue to function long enough to get some useful work done. For the first time, there was a clear separation between designers, builders, operators, programmers, and maintenance personnel.
These machines, now called mainframes, were locked away in specially air-conditioned computer rooms, with staffs of specially-trained professional operators to run them. Only big corporations or major government agencies or universities could afford their multimillion dollar price tags. To run a job (i.e., a program or set of programs), a programmer would first write the program on paper (in FORTRAN or possibly even in assembly language), then punch it on cards. He would then bring the card deck down to the input room and hand it to one of the operators and go drink coffee until the output was ready.
When the computer finished whatever job it was currently running, an operator would go over to the printer and tear off the output and carry it over to the output-room, so that the programmer could collect it later. Then he would take one of the card decks that had been brought from the input room and read it in. If the FORTRAN compiler was needed, the operator would have to get it from a file cabinet and read it in. Much computer time was wasted while operators were walking around the machine room.
Given the high cost of the equipment, it is not surprising that people quickly looked for ways to reduce the wasted time. The solution generally adopted was the batch system. The idea behind it was to collect a tray full of jobs in the input room and then read them onto a magnetic tape using a small (relatively) inexpensive computer, such as the IBM 1401, which was very good at reading cards, copying tapes, and printing output, but not at all good at numerical calculations. Other, much more expensive machines, such as the IBM 7094, were used for the real computing. This situation is shown in Fig. 1-2.
After about an hour of collecting a batch of jobs, the tape was rewound and brought into the machine room, where it was mounted on a tape drive. The operator then loaded a special program (the ancestor of today's operating system), which read the first job from tape and ran it. The output was written onto a second tape, instead of being printed. After each job finished, the operating system automatically read the next job from the tape and began running it. When the whole batch was done, the operator removed the input and output tapes, replaced the input tape with the next batch, and brought the output tape to a 1401 for printing off line (i.e., not connected to the main computer).
The structure of a typical input job is shown in Fig. 1-3. It started out with a $JOB card, specifying the maximum run time in minutes, the account number to be charged, and the programmer's name. Then came a $FORTRAN card, telling the operating system to load the FORTRAN compiler from the system tape. It was followed by the program to be compiled, and then a $LOAD card, directing the operating system to load the object program just compiled. (Compiled programs were often written on scratch tapes and had to be loaded explicitly.) Next came the $RUN card, telling the operating system to run the program with the data following it. Finally, the $END card marked the end of the job. These primitive control cards were the forerunners of modern job control languages and command interpreters.
Large second-generation computers were used mostly for scientific and engineering calculations, such as solving the partial differential equations that often occur in physics and engineering. They were largely programmed in FORTRAN and assembly language. Typical operating systems were FMS (the Fortran Monitor System) and IBSYS, IBM's operating system for the 7094.

The Third Generation (1965-80) ICs and Multiprogramming
By the early 1960s, most computer manufacturers had two distinct, and totally incompatible, product lines. On the one hand there were the word-oriented, large-scale scientific computers, such as the 7094, which were used for numerical calculations in science and engineering. On the other hand, there were the character-oriented, commercial computers, such as the 1401, which were widely used for tape sorting and printing by banks and insurance companies.
Developing, maintaining, and marketing two completely different product lines was an expensive proposition for the computer manufacturers. In addition, many new computer customers initially needed a small machine but later outgrew it and wanted a bigger machine that had the same architectures as their current one so it could run all their old programs, but faster. IBM attempted to solve both of these problems at a single stroke by introducing the System/360. The 360 was a series of software-compatible machines ranging from 1401-sized to much more powerful than the 7094. The machines differed only in price and performance (maximum memory, processor speed, number of I/O devices permitted, and so forth). Since all the machines had the same architecture and instruction set, programs written for one machine could run on all the others, at least in theory. Furthermore, the 360 was designed to handle both scientific (i.e., numerical) and commercial computing. Thus a single family of machines could satisfy the needs of all customers.
In subsequent years, IBM has come out with compatible successors to the 360 line, using more modern technology, known as the 370, 4300, 3080, 3090, and Z series.
The 360 was the first major computer line to use (small-scale) Integrated Circuits (ICs), thus providing a major price/performance advantage over the second-generation machines, which were built up from individual transistors. It was an immediate success, and the idea of a family of compatible computers was soon adopted by all the other major manufacturers. The descendants of these machines are still in use at computer centers today(1996). Nowadays they are often used for managing huge databases (e.g., for airline reservation systems) or as servers for World Wide Web sites that must process thousands of requests per second.
The greatest strength of the "one family" idea was simultaneously its greatest weakness. The intention was that all software, including the operating system, OS/360 , had to work on all models. It had to run on small systems, which often just replaced 1401s for copying cards to tape, and on very large systems, which often replaced 7094s for doing weather forecasting and other heavy computing. It had to be good on systems with few peripherals and on systems with many peripherals. It had to work in commercial environments and in scientific environments. Above all, it had to be efficient for all of these different uses.
There was no way that IBM (or anybody else) could write a piece of software to meet all those conflicting requirements. The result was an enormous and extraordinarily complex operating system, probably two to three orders of magnitude larger than FMS. It consisted of millions of lines of assembly language written by thousands of programmers, and contained thousands upon thousands of bugs, which necessitated a continuous stream of new releases in an attempt to correct them. Each new release fixed some bugs and introduced new ones, so the number of bugs probably remained constant in time.
One of the designers of OS/360, Fred Brooks, subsequently wrote a witty and incisive book describing his experiences with OS/360 (Brooks, 1995). Suffice it to say that the cover shows a herd of prehistoric beasts stuck in a tar pit. The cover of Silberschatz et al. (2004) makes a similar point about operating systems being dinosaurs.
Despite its enormous size and problems, OS/360 and the similar third-generation operating systems produced by other computer manufacturers actually satisfied most of their customers reasonably well. They also popularized several key techniques absent in second-generation operating systems. Probably the most important of these was multiprogramming. On the 7094, when the current job paused to wait for a tape or other I/O operation to complete, the CPU simply sat idle until the I/O finished. With heavily CPU-bound scientific calculations, I/O is infrequent, so this wasted time is not significant. With commercial data processing, the I/O wait time can often be 80 or 90 percent of the total time, so something had to be done to avoid having the (expensive) CPU be idle so much.

The solution that evolved was to partition memory into several pieces, with a different job in each partition, as shown in Fig. 1-4. While one job was waiting for I/O to complete, another job (just sited in RAM) could be using the CPU. If enough jobs could be held in main memory at once, the CPU could be kept busy nearly 100 percent of the time regard I/O time delays. Having multiple jobs safely in memory at once requires special hardware to protect each job against snooping and mischief by the other ones, but the 360 and other third-generation systems were equipped with this hardware.
Another major feature present in third-generation operating systems was the ability to read jobs from cards onto the disk as soon as they were brought to the computer room. Then, whenever a running job finished, the operating system could load a new job from the disk into the now-empty RAM partition and run it. This technique is called spooling (Simultaneous Peripheral Operation On Line) and was also used for output. With spooling, the 1401s were no longer needed, and much carrying of tapes disappeared.
Although third-generation operating systems were well suited for big scientific calculations and massive commercial data processing runs, they were still basically batch systems. Many programmers pined for the first-generation days when they had the machine all to themselves for a few hours, so they could debug their programs quickly. With third-generation systems, the time between submitting a job and getting back the output was often hours, so a single misplaced comma could cause a compilation to fail, and the programmer to waste half a day. This desire for quick response time paved the way for timesharing, a variant of multiprogramming, in which each user has an online terminal. In a timesharing system, if 20 users are logged in and 17 of them are thinking or talking or drinking coffee, the CPU can be allocated in turn to the three jobs that want service. Since people debugging programs usually issue short commands (e.g., compile a five-page procedure[We will use the terms "procedure," "subroutine," and "function" interchangeably here]) rather than long ones (e.g., sort a million-record file), the computer can provide fast, interactive service to a number of users and perhaps also work on big batch jobs in the background when the CPU is otherwise idle. The first serious timesharing system, CTSS (Compatible Time Sharing System), was developed at M.I.T. on a specially modified 7094 (Corbató et al., 1962). However, timesharing did not really become popular until the necessary protection hardware became widespread during the third generation.
After the success of the CTSS system, MIT, Bell Labs, and General Electric (then a major computer manufacturer) decided to embark on the development of a "computer utility," a machine that would support hundreds of simultaneous timesharing users. Their model was the electricity distribution system when you need electric power, you just stick a plug in the wall, and within reason, as much power as you need will be there. The designers of this system, known as MULTICS (MULTiplexed Information and Computing Service), envisioned one huge machine providing computing power for everyone in the Boston area. The idea that machines far more powerful than their GE-645 mainframe would be sold for under a thousand dollars by the millions only 30 years later was pure science fiction.
MULTICS was a mixed success. It was designed to support hundreds of users on a machine only slightly more powerful than an Intel 80386-based PC, although it had much more I/O capacity. This is not quite as crazy as it sounds, since people knew how to write small, efficient programs in those days, a skill that has subsequently been lost. There were many reasons that MULTICS did not take over the world, not the least of which is that it was written in PL/I, and the PL/I compiler was years late and barely worked at all when it finally arrived. In addition, MULTICS was enormously ambitious for its time, much like Charles Babbage's analytical engine in the nineteenth century.
MULTICS introduced many seminal ideas into the computer literature, but turning it into a serious product and a commercial success was a lot harder than anyone had expected. Bell Labs dropped out of the project, and General Electric quit the computer business altogether. However, M.I.T. persisted and eventually got MULTICS working. It was ultimately sold as a commercial product by the company that bought GE's computer business (Honeywell) and installed by about 80 major companies and universities worldwide. While their numbers were small, MULTICS users were fiercely loyal. General Motors, Ford, and the U.S. National Security Agency, for example, only shut down their MULTICS systems in the late 1990s. The last MULTICS running, at the Canadian Department of National Defence, shut down in October 2000. Despite its lack of commercial success, MULTICS had a huge influence on subsequent operating systems. A great deal of information about it exists (Corbató et al., 1972; Corbató and Vyssotsky, 1965; Daley and Dennis, 1968; Organick, 1972; and Saltzer, 1974). It also has a still active Web site, with a great deal of information about the system, its designers, and its users.
The phrase "computer utility" is no longer heard, but the idea has gained new life in recent years. In its simplest form, PCs or workstations (high-end PCs) in a business or a classroom may be connected via a LAN (Local Area Network) to a file server on which all programs and data are stored. An administrator then has to install and protect only one set of programs and data, and can easily reinstall local software on a malfunctioning PC or workstation without worrying about retrieving or preserving local data. In more heterogeneous environments, a class of software called middleware has evolved to bridge the gap between local users and the files, programs, and databases they use on remote servers. Middleware makes networked computers look local to individual users' PCs or workstations and presents a consistent user interface even though there may be a wide variety of different servers, PCs, and workstations in use. The World Wide Web is an example. A web browser presents documents to a user in a uniform way, and a document as seen on a user's browser can consist of text from one server and graphics from another server, presented in a format determined by a style sheet on yet another server. Businesses and universities commonly use a web interface to access databases and run programs on a computer in another building or even another city. Middleware appears to be the operating system of a distributed system, but it is not really an operating system at all, and is beyond our scope here[*].
[*] For more on distributed systems see Tanenbaum and Van Steen (2002).
Another major development during the third generation was the phenomenal growth of minicomputers, starting with the Digital Equipment Company (DEC) PDP-1 in 1961. The PDP-1 had only 4K of 18-bit words, but at $120,000 per machine (less than 5 percent of the price of a 7094), it sold like hotcakes. For certain kinds of non numerical work, it was almost as fast as the 7094 and gave birth to a whole new industry. It was quickly followed by a series of other PDPs (unlike IBM's family, all incompatible) culminating in the PDP-11.
One of the computer scientists at Bell Labs who had worked on the MULTICS project, Ken Thompson, subsequently found a small PDP-7 minicomputer that no one was using and set out to write a stripped-down, one-user version of MULTICS. This work later developed into the UNIX operating system, which became popular in the academic world, with government agencies, and with many companies.
The history of UNIX has been told elsewhere (e.g., Salus, 1994). Because the source code was widely available, various organizations developed their own (incompatible) versions, which led to chaos. Two major versions developed, System V, from AT&T, and BSD, (Berkeley Software Distribution) from the University of California at Berkeley. These had minor variants as well, now including FreeBSD, OpenBSD, and NetBSD. To make it possible to write programs that could run on any UNIX system, IEEE developed a standard for UNIX, called POSIX, that most versions of UNIX now support. POSIX defines a minimal system call interface that conformant UNIX systems must support. In fact, some other operating systems now also support the POSIX interface. The information needed to write POSIX-compliant software is available in books (IEEE, 1990; Lewine, 1991), and online as the Open Group's "Single UNIX Specification". When we refer to UNIX, we mean all of these systems as well, unless stated otherwise. While they differ internally, all of them support the POSIX standard, so to the programmer they are quite similar.

The Fourth Generation (1980-Present) Personal Computers
With the development of LSI (Large Scale Integration) circuits, chips containing thousands of transistors on a square centimeter of silicon(nowadays houndreds of millions per single IC package) , the age of the microprocessor-based personal computer dawned. In terms of architecture, personal computers (initially called microcomputers) were not all that different from minicomputers of the PDP-11 class, but in terms of price they certainly were different. The minicomputer made it possible for a department in a company or university to have its own computer. The microcomputer made it possible for an individual to have his or her own computer.
There were several families of microcomputers. Intel came out with the 8080, the first general-purpose 8-bit microprocessor, in 1974. A number of companies produced complete systems using the 8080 (or the compatible Zilog Z80) and the CP/M (Control Program for Microcomputers) operating system from a company called Digital Research was widely used with these. Many application programs were written to run on CP/M, and it dominated the personal computing world for about 5 years.
Motorola also produced an 8-bit microprocessor, the 6800. A group of Motorola engineers left to form MOS Technology and manufacture the 6502 CPU after Motorola rejected their suggested improvements to the 6800. The 6502 was the CPU of several early systems. One of these, the Apple II, became a major competitor for CP/M systems in the home and educational markets. But CP/M was so popular that many owners of Apple II computers purchased Z-80 coprocessor add-on cards to run CP/M, since the 6502 CPU was not compatible with CP/M. The CP/M cards were sold by a little company called Microsoft, which also had a market niche supplying BASIC interpreters used by a number of microcomputers running CP/M.
The next generation of microprocessors were 16-bit systems. Intel came out with the 8086, and in the early 1980s, IBM designed the IBM PC around Intel's 8088 (an 8086 on the inside, with an 8 bit external data path). Microsoft offered IBM a package which included Microsoft's BASIC and an operating system, DOS (Disk Operating System) originally developed by another company Microsoft bought the product and hired the original author to improve it. The revised system was renamed MS-DOS (MicroSoft Disk Operating System) and quickly came to dominate the IBM PC market.
CP/M, MS-DOS, and the Apple DOS were all command-line systems: users typed commands at the keyboard. Years earlier, Doug Engelbart at Stanford Research Institute had invented the GUI (Graphical User Interface), pronounced "gooey," complete with windows, icons, menus, and mouse. Apple's Steve Jobs saw the possibility of a truly user-friendly personal computer (for users who knew nothing about computers and did not want to learn), and the Apple Macintosh was announced in early 1984. It used Motorola's 16-bit 68000 CPU, and had 64 KB of ROM (Read Only Memory), to support the GUI. The Macintosh has evolved over the years. Subsequent Motorola CPUs were true 32-bit systems, and later still Apple moved to IBM PowerPC CPUs, with RISC 32-bit (and later, 64-bit) architecture. In 2001 Apple made a major operating system change, releasing Mac OS X, with a new version of the Macintosh GUI on top of Berkeley UNIX. And in 2005 Apple announced that it would be switching to Intel processors.
To compete with the Macintosh, Microsoft invented Windows. Originally Windows was just a graphical environment on top of 16-bit MS-DOS (i.e., it was more like a shell than a true operating system). However, current versions of Windows are descendants of Windows NT, a full 32-bit system, rewritten from scratch.
The other major contender in the personal computer world is UNIX (and its various derivatives). UNIX is strongest on workstations and other high-end computers, such as network servers. It is especially popular on machines powered by high-performance RISC chips. On Pentium-based computers, Linux is becoming a popular alternative to Windows for students and increasingly many corporate users. ( we will use the term "Pentium" to mean the entire Pentium family, including the low-end Celeron, the high end Xeon, and compatible AMD microprocessors).
Although many UNIX users, especially experienced programmers, prefer a command-based interface to a GUI, nearly all UNIX systems support a windowing system called the X Window system developed at M.I.T. This system handles the basic window management, allowing users to create, delete, move, and resize windows using a mouse. Often a complete GUI, such as Motif, GNOME, KDE is available to run on top of the X Window system giving UNIX a look and feel something like the Macintosh or Microsoft Windows for those UNIX users who want such a thing.
An interesting development that began taking place during the mid-1980s is the growth of networks of personal computers running network operating systems and distributed operating systems (Tanenbaum and Van Steen, 2002).

In a network operating system, the users are aware of the existence of multiple computers and can log in to remote machines and copy files from one machine to another. Each machine runs its own local operating system and has its own local user (or users). Basically, the machines are independent of one another. Network operating systems are not fundamentally different from single-processor operating systems. They obviously need a network interface controller and some low-level software to drive it, as well as programs to achieve remote login and remote file access, but these additions do not change the essential structure of the operating system.

A distributed operating system, in contrast, is one that appears to its users as a traditional uniprocessor system, even though it is actually composed of multiple processors. The users should not be aware of where their programs are being run or where their files are located; that should all be handled automatically and efficiently by the operating system.
True distributed operating systems require more than just adding a little code to a uniprocessor operating system, because distributed and centralized systems differ in critical ways. Distributed systems, for example, often allow applications to run on several processors at the same time, thus requiring more complex processor scheduling algorithms in order to optimize the amount of parallelism. Communication delays within the network often mean that these (and other) algorithms must run with incomplete, outdated, or even incorrect information. This situation is radically different from a single-processor system in which the operating system has complete information about the system state.

History of LINUX®
When UNIX was young (UNIX Version 6), the source code was widely available, under AT&T license, and frequently studied. John Lions, of the University of New South Wales in Australia, even wrote a little booklet describing its operation, line by line (Lions, 1996). This booklet was used (with permission of AT&T) as a text in many university operating system courses.
When AT&T released Version 7, it dimly began to realize that UNIX was a valuable commercial product, so it issued Version 7 with a license that prohibited the source code from being studied in courses, in order to avoid endangering its status as a trade secret. Many universities complied by simply dropping the study of UNIX and teaching only theory.
Unfortunately, teaching only theory leaves the student with a lopsided view of what an operating system is really like. The theoretical topics that are usually covered in great detail in courses and books on operating systems, such as scheduling algorithms, are in practice not really that important. Subjects that really are important, such as I/O and file systems, are generally neglected because there is little theory about them.
To remedy this situation Tanenbaum decided to write a new operating system from scratch that would be compatible with UNIX from the user's point of view, but completely different on the inside. By not using even one line of AT&T code, this system avoided the licensing restrictions, so it could be used for class or individual study. In this manner, readers could dissect a real operating system to see what is inside, just as biology students dissect frogs. It was called MINIX and was released in 1987 with its complete source code for anyone to study or modify. The name MINIX stands for mini-UNIX because it is small enough that even a non guru can understand how it works.
In addition to the advantage of eliminating the legal problems, MINIX had another advantage over UNIX. It was written a decade after UNIX and was structured in a more modular way. For instance, from the very first release of MINIX the file system and the memory manager were not part of the operating system at all but ran as user programs. In the current release (MINIX 3) this modularization has been extended to the I/O device drivers, which (with the exception of the clock driver) all run as user programs. Another difference is that UNIX was designed to be efficient; MINIX was designed to be readable (in as much as one can speak of any program hundreds of pages long as being readable). The MINIX code, for example, has thousands of comments in it.
MINIX was originally designed for compatibility with Version 7 (V7) UNIX. Version 7 was used as the model because of its simplicity and elegance. It is sometimes said that Version 7 was an improvement not only over all its predecessors, but also over all its successors. With the advent of POSIX, MINIX began evolving toward the new standard, while maintaining backward compatibility with existing programs. This kind of evolution is common in the computer industry, as no vendor wants to introduce a new system that none of its existing customers can use without great upheaval. So MINIX 3, is based on the POSIX standard.
Like UNIX, MINIX was written in the C programming language and was intended to be easy to port to various computers. The initial implementation was for the IBM PC. MINIX was subsequently ported to several other platforms. In keeping with the "Small is Beautiful" philosophy, MINIX originally did not even require a hard disk to run (in the mid-1980s hard disks were still an expensive novelty). As MINIX grew in functionality and size, it eventually got to the point that a hard disk was needed for PCs, but in keeping with the MINIX philosophy, a 200-MB partition is sufficient (for embedded applications, no hard disk is required though). In contrast, even small Linux systems require 500-MB of disk space, and several GB will be needed to install common applications.
To the average user sitting at an IBM PC, running MINIX is similar to running UNIX. All of the basic programs, such as cat, grep, ls, make, and the shell are present and perform the same functions as their UNIX counterparts. Like the operating system itself, all these utility programs have been rewritten completely from scratch by Tanenbaum, his students, and some other dedicated people, with no AT&T or other proprietary code. Many other freely-distributable programs now exist, and in many cases these have been successfully ported (recompiled) on MINIX.
MINIX continued to develop for a decade and MINIX 2 was released in 1997, together with the second edition of Tanenbaums book, which described the new release. The changes between versions 1 and 2 were substantial (e.g., from 16-bit real mode on an 8088 using floppy disks to 32-bit protected mode on a 386 using a hard disk) but evolutionary.
Development continued slowly but systematically until 2004, when Tanenbaum became convinced that software was getting too bloated and unreliable and decided to pick up the slightly-dormant MINIX thread again. Together with his students and programmers at the Vrije Universiteit in Amsterdam, he produced MINIX 3, a major redesign of the system, greatly restructuring the kernel, reducing its size, and emphasizing modularity and reliability. The new version was intended both for PCs and embedded systems, where compactness, modularity, and reliability are crucial. While some people in the group called for a completely new name, it was eventually decided to call it MINIX 3 since the name MINIX was already well known. By way of analogy, when Apple abandoned it own operating system, Mac OS 9 and replaced it with a variant of Berkeley UNIX, the name chosen was Mac OS X rather than APPLIX or something like that. Similar fundamental changes have happened in the Windows family while retaining the Windows name.
The MINIX 3 kernel is well under 4000 lines of executable code, compared to millions of executable lines of code for Windows, Linux, FreeBSD, and other operating systems. Small kernel size is important because kernel bugs are far more devastating than bugs in user-mode programs and more code means more bugs. One careful study has shown that the number of detected bugs per 1000 executable lines of code varies from 6 to 16 (Basili and Perricone, 1984). The actual number of bugs is probably much higher since the researchers could only count reported bugs, not unreported bugs. Yet another study (Ostrand et al., 2004) showed that even after more than a dozen releases, on the average 6% of all files contained bugs that were later reported and after a certain point the bug level tends to stabilize rather than go asymptotically to zero. This result is supported by the fact that when a very simple, automated, model-checker was let loose on stable versions of Linux and OpenBSD, it found hundreds of kernel bugs, overwhelmingly in device drivers (Chou et al., 2001; and Engler et al., 2001). This is the reason the device drivers were moved out of the kernel in MINIX 3; they can do less damage in user mode.
Most of the comments about the MINIX 3 system calls, however (as opposed to comments about the actual code), also apply to other UNIX systems.

A few words about Linux and its relationship to MINIX: Shortly after MINIX was released, a USENET newsgroup, comp.os.minix, was formed to discuss it. Within weeks, it had 40,000 subscribers, most of whom wanted to add vast numbers of new features to MINIX to make it bigger and better (well, at least bigger). Every day, several hundred of them offered suggestions, ideas, and frequently snippets of source code. The author of MINIX was able to successfully resist this onslaught for several years, in order to keep MINIX clean enough for students to understand and small enough that it could run on computers that students could afford. For people who thought little of MS-DOS, the existence of MINIX (with source code) as an alternative was even a reason to finally go out and buy a PC.
One of these people was a Finnish student named Linus Torvalds. Torvalds installed MINIX on his new PC and studied the source code carefully. Torvalds wanted to read USENET newsgroups (such as comp.os.minix) on his own PC rather than at his university, but some features he needed were lacking in MINIX, so he wrote a program to do that, but soon discovered he needed a different terminal driver, so he wrote that too. Then he wanted to download and save postings, so he wrote a disk driver, and then a file system. By Aug. 1991 he had produced a primitive kernel. On Aug. 25, 1991, he announced it on comp.os.minix. This announcement attracted other people to help him, and on March 13, 1994 Linux 1.0 was released. Thus was Linux born.
Linux has become one of the notable successes of the open source movement (which MINIX helped start). Linux is challenging UNIX (and Windows) in many environments, partly because commodity PCs which support Linux are now available with performance that rivals the proprietary RISC systems required by some UNIX implementations. Other open source software, notably the Apache web server and the MySQL database, work well with Linux in the commercial world. Linux, Apache, MySQL, and the open source Perl and PHP programming languages are often used together on web servers and are sometimes referred to by the acronym LAMP. For more on the history of Linux and open source software see DiBona et al. (1999), Moody (2001), and Naughton (2000).

So spesifically Linux [LINUX® is a registered trademark of Linus Torvalds] is a member of the large family of Unix-like operating systems . A relative newcomer experiencing sudden spectacular popularity starting in the late 1990s, Linux joins such well-known commercial Unix operating systems as System V Release 4 (SVR4), developed by AT&T (now owned by the SCO Group); the 4.4 BSD release from the University of California at Berkeley (4.4BSD); Digital UNIX from Digital Equipment Corporation (now Hewlett-Packard); AIX from IBM; HP-UX from Hewlett-Packard; Solaris from Sun Microsystems; and Mac OS X from Apple Computer Inc. Beside Linux, a few other opensource Unix(BSD)-like kernels exist, such as FreeBSD , NetBSD , and OpenBSD .

Linux was initially developed by Linus Torvalds in 1991 as an operating system for IBM -compatible personal computers based on the Intel 80386 microprocessor. Linus remains deeply involved with improving Linux, keeping it up-to-date with various hardware developments and coordinating the activity of hundreds of Linux developers around the world. Over the years, developers have worked to make Linux available on other architectures, including Hewlett-Packard's (ex DEC's) Alpha, Intel's Itanium, AMD's AMD64,IBM's PowerPC, and zSeries.

One of the more appealing benefits to Linux is that it isn't a commercial operating system: its source code under the GNU General Public License (GPL) [*] is open and available to anyone to study; if you download the kernel source code or check the sources on a Linux CD, you will be able to explore, from top to bottom, one of the most successful modern operating systems. Here, in fact, i assume you have the source code on hand and can apply what i say to your own explorations.
[*] The GNU project is coordinated by the Free Software Foundation; its aim is to implement a whole operating system freely usable by everyone. The availability of a GNU C compiler has been essential for the success of the Linux project.
Technically speaking, Linux is a true Unix kernel, although it is not a full Unix operating system because it does not include all the Unix applications, such as filesystem utilities, windowing systems and graphical desktops, system administrator commands, text editors, compilers, and so on. However, because most of these programs are freely available under the GPL, they can be installed in every Linux-based system.

Because the Linux kernel requires so much additional software to provide a useful environment, many Linux users prefer to rely on commercial distributions(distros), available on CD-ROM, or on Internet to get the code included in a standard Unix system. Alternatively, the code may be obtained from several different sites( for instance LFS, . Several distributions put the Linux source code in the /usr/src/linux directory. In the rest of this post, all file pathnames will refer implicitly to the Linux source code directory.

Linux Versus Other Unix-Like Kernels
The various Unix-like systems on the market, some of which have a long history and show signs of archaic practices, differ in many important respects. All commercial variants were derived from either SVR4 or 4.4BSD, and all tend to agree on some common standards like IEEE's Portable Operating Systems based on Unix (POSIX) and X/Open's Common Applications Environment (CAE).

The current standards specify only an application programming interface (API) that is, a well-defined environment in which user programs should run. Therefore, the standards do not impose any restriction on internal design choices of a compliant kernel.(As a matter of fact, several non-Unix operating systems, such as Windows NT and its descendents, are POSIX-compliant)

To define a common user interface, Unix-like kernels often share fundamental design ideas and features. In this respect, Linux is comparable with the other Unix-like operating systems. Reading this text and studying the Linux kernel, therefore, may help you understand the other Unix variants, too.

The 2.6 version of the Linux kernel aims to be compliant with the IEEE POSIX standard. This, of course, means that most existing Unix programs can be compiled and executed on a Linux system with very little effort or even without the need for patches to the source code. Moreover, Linux includes all the features of a modern Unix operating system, such as virtual memory, a virtual filesystem, lightweight processes, Unix signals , SVR4 interprocess communications, support for Symmetric Multiprocessor (SMP) systems, and so on.

When Linus Torvalds wrote the first kernel, he referred to some classical books on Unix internals, like Maurice Bach's The Design of the Unix Operating System (Prentice Hall, 1986). Actually, Linux still has some bias toward the Unix baseline described in Bach's book (i.e., SVR2). However, Linux doesn't stick to any particular variant. Instead, it tries to adopt the best features and design choices of several different Unix kernels.

The following list describes how Linux competes against some well-known commercial Unix kernels:
  • Monolithic kernel It is a large, complex do-it-yourself program, composed of several logically different components. In this, it is quite conventional; most commercial Unix variants are monolithic. (Notable exceptions are the Apple's Mac OS X and the GNU Hurd operating systems, both derived from the Carnegie-Mellon's Mach, which follow a microkernel approach.)
  • Compiled and statically linked traditional Unix kernels Most modern kernels can dynamically load and unload some portions of the kernel code (typically, device drivers), which are usually called modules . Linux's support for modules is very good, because it is able to automatically load and unload modules on demand. Among the main commercial Unix variants, only the SVR4.2 and Solaris kernels have a similar feature.
  • Kernel threading Some Unix kernels, such as Solaris and SVR4.2/MP, are organized as a set of kernel threads . A kernel thread is an execution context that can be independently scheduled; it may be associated with a user program, or it may run only some kernel functions. Context switches between kernel threads are usually much less expensive than context switches between ordinary processes, because the former usually operate on a common address space. Linux uses kernel threads in a very limited way to execute a few kernel functions periodically; however, they do not represent the basic Linux execution context abstraction. (That's the topic of the next item.)
  • Multithreaded application support Most modern operating systems have some kind of support for multithreaded applications that is, user programs that are designed in terms of many relatively independent execution flows that share a large portion of the application data structures. A multithreaded user application could be composed of many lightweight processes (LWP), which are processes that can operate on a common address space, common physical memory pages, common opened files, and so on. Linux defines its own version of lightweight processes, which is different from the types used on other systems such as SVR4 and Solaris. While all the commercial Unix variants of LWP are based on kernel threads, Linux regards lightweight processes as the basic execution context and handles them via the non standard clone( ) system call.
  • Preemptive kernel When compiled with the "Preemptible Kernel" option, Linux 2.6 can arbitrarily interleave execution flows while they are in privileged mode. Besides Linux 2.6, a few other conventional, general-purpose Unix systems, such as Solaris and Mach 3.0 , are fully preemptive kernels. SVR4.2/MP introduces some fixed preemption points as a method to get limited preemption capability.
  • Multiprocessor support Several Unix kernel variants take advantage of multiprocessor systems. Linux 2.6 supports symmetric multiprocessing (SMP ) for different memory models, including NUMA: the system can use multiple processors and each processor can handle any task there is no discrimination among them. Although a few parts of the kernel code are still serialized by means of a single "big kernel lock ," it is fair to say that Linux 2.6 makes a near optimal use of SMP.
  • STREAMS Linux has no analog to the STREAMS I/O subsystem introduced in SVR4, although it is included now in most Unix kernels and has become the preferred interface for writing device drivers, terminal drivers, and network protocols.
This assessment suggests that Linux is fully competitive nowadays with commercial operating systems. Moreover, Linux has several features that make it an exciting operating system(at least for studying till production purposes). Commercial Unix kernels often introduce new features to gain a larger slice of the market, but these features are not necessarily useful, stable, or productive. As a matter of fact, modern Unix kernels tend to be quite bloated. By contrast, Linux together with the other open source operating systems doesn't suffer from the restrictions and the conditioning imposed by the market, hence it can freely evolve according to the ideas of its designers (mainly Linus Torvalds).

Specifically, Linux offers the following advantages over its commercial competitors:
  • Linux is cost-free You can install a complete Unix system at no expense other than the hardware (it is hoped compatible).
  • Linux is fully customizable in all its components Thanks to the compilation options, you can customize the kernel by selecting only the features really needed. Moreover, thanks to the GPL, you are allowed to freely read and modify the source code of the kernel and of all system programs(it is hoped you're capable).[*]
    [*] Many commercial companies are now supporting their products under Linux. However, many of them aren't distributed under an open source license, so you might not be allowed to read or modify their source code.
  • Linux runs on low-end, inexpensive hardware platforms You are able to build a network server using an old Intel 80386 system with 4 MB of RAM.
  • Linux is powerful Linux systems tend to be and in fact are very fast, because they fully exploit the features of the hardware components. The main Linux goal is efficiency, and indeed many design choices of commercial variants, like the STREAMS I/O subsystem, have been rejected by Linus because of their implied performance penalty.
  • Linux developers are excellent programmers Linux systems are very stable; they have a very low failure rate and system maintenance time.
  • The Linux kernel can be very small and compact It is possible to fit a kernel image, including a few system programs, on just one 1.44 MB floppy disk. As far as we know, none of the commercial Unix variants is able to boot from a single floppy disk.
  • Linux is well supported Believe it or not, it may be a lot easier to get patches and updates for Linux than for any proprietary operating system. The answer to a problem often comes back within a few hours after sending a message to some newsgroup or mailing list. Moreover, drivers(from community) for Linux are usually available a few weeks after new hardware products have been introduced on the market. By contrast, hardware manufacturers release device drivers for only a few commercial operating systems usually Microsoft's. Therefore, all commercial Unix variants run on a restricted subset of hardware components(one of the main reasons because Linus start).
With an estimated installed base of several hundreds of millions, people who are used to certain features that are standard under other operating systems are starting to expect the same from Linux. In that regard, the demand on Linux developers is also increasing. Luckily, though, Linux has evolved under the close direction of Linus and his subsystem maintainers to accommodate the needs of the masses.

Hardware Dependency
Linux tries to maintain a neat distinction between hardware-dependent and hardware-independent source code. To that end, both the arch and the include directories include 23 subdirectories that correspond to the different types of hardware platforms supported. The standard names of the platforms (in italic the most common) are:
  • alpha Hewlett-Packard's Alpha workstations (originally Digital, then Compaq; no longer manufactured)
  • arm, arm26 ARM processor-based computers such as PDAs and embedded devices
  • cris "Code Reduced Instruction Set" CPUs used by Axis in its thin-servers, such as web cameras or development boards
  • frv Embedded systems based on microprocessors of the Fujitsu's FR-V family
  • i386 IBM-compatible personal computers based on 80x86 microprocessors
  • ia64 Workstations based on the Intel 64-bit Itanium microprocessor
  • ppc, ppc64 Workstations based on the 32-bit and 64-bit Motorola-IBM PowerPC microprocessors
  • sparc, sparc64 Workstations based on Sun Microsystems SPARC and 64-bit Ultra SPARC microprocessors
  • um User Mode Linux, a virtual platform that allows developers to run a kernel in User Mode

Linux Versions
Up to kernel version 2.5, Linux identified kernels through a simple numbering scheme. Each version was characterized by three numbers, separated by periods. The first two numbers were used to identify the version; the third number identified the release. The first version number, namely 2, has stayed unchanged since 1996. The second version number identified the type of kernel: if it was even, it denoted a stable version; otherwise, it denoted a development version.
As the name suggests, stable versions were thoroughly checked by Linux distributors and kernel hackers. A new stable version was released only to address bugs and to add new device drivers. Development versions, on the other hand, differed quite significantly from one another; kernel developers were free to experiment with different solutions that occasionally lead to drastic kernel changes. Users who relied on development versions for running applications could experience unpleasant surprises when upgrading their kernel to a newer release.

During development of Linux kernel version 2.6, however, a significant change in the version numbering scheme has taken place. Basically, the second number no longer identifies stable or development versions; thus, nowadays kernel developers introduce large and significant changes in the current kernel version 2.6. A new kernel 2.7 branch will be created only when kernel developers will have to test a really disruptive change; this 2.7 branch will lead to:
  • a new current kernel version, or
  • it will be backported to the 2.6 version, or
  • finally it will simply be dropped as a dead end.
The new model of Linux development implies that two kernels having the same version but different release numbers for instance, 2.6.10 and 2.6.11can differ significantly even in core components and in fundamental algorithms. Thus, when a new kernel release appears, it is potentially unstable and buggy. To address this problem, the kernel developers may release patched versions of any kernel, which are identified by a fourth number in the version numbering scheme. For instance, at the time this paragraph was written, the latest "stable" kernel version was

Basic Operating System Concepts
Each computer system includes a basic set of programs called the operating system. Operating systems typically have four major components:
  • process management
  • I/O device management
  • memory management and
  • file management.
The most important program in the set is called the kernel. It is loaded into RAM when the system boots and contains many critical procedures that are needed for the system to operate. The other programs are less crucial utilities; they can provide a wide variety of interactive experiences for the user as well as doing all the jobs the user bought the computer for but the essential shape and capabilities of the system are determined by the kernel. The kernel provides key facilities to everything else on the system and determines many of the characteristics of higher level software. Hence, we often use the term "operating system" as a synonym for "kernel."

The operating system must fulfill two main objectives:

  • Interact with the hardware components, servicing all low-level programmable elements included in the hardware platform.
  • Provide an execution environment to the applications that run on the computer system (the so-called user programs).
Some operating systems allow all user programs to directly play with the hardware components (a typical example is MS-DOS ). In contrast, a Unix-like operating system hides all low-level details concerning the physical organization of the computer from applications run by the user. When a program wants to use a hardware resource:
  1. it must issue a request to the operating system.
  2. The kernel evaluates the request and,
  3. if it chooses to grant the resource, interacts with the proper hardware components on behalf of the user program.
To enforce this mechanism, modern operating systems rely on the availability of specific hardware features that forbid user programs to directly interact with low-level hardware components or to access arbitrary memory locations. In particular, the hardware introduces at least two different execution modes for the CPU: a non-privileged mode for user programs and a privileged mode for the kernel. Unix calls these User Mode and Kernel Mode , respectively.
In the rest of this section, we introduce the basic concepts that have motivated the design of Unix over the past two decades, as well as Linux and other operating systems. While the concepts are probably familiar to you as a Linux user, these sections try to delve into them a bit more deeply than usual to explain the requirements they place on an operating system kernel. These broad considerations refer to virtually all Unix-like systems. The other sections of this post will hopefully help you understand the specific Linux kernel internals.

Multiuser Systems

A multiuser system is a computer that is able to concurrently and independently execute several applications belonging to two or more users. Concurrently means that applications can be active at the same time and contend for the various resources such as CPU, memory, hard disks, and so on. Independently means that each application can perform its task with no concern for what the applications of the other users are doing. Switching from one application to another, of course, slows down each of them and affects the response time seen by the users. Many of the complexities of modern operating system kernels, which we will examine here, are present to minimize the delays enforced on each program and to provide the user with responses that are as fast as possible.

Multiuser operating systems must include several features:
  • An authentication mechanism for verifying the user's identity
  • A protection mechanism against buggy user programs that could block other applications running in the system
  • A protection mechanism against malicious user programs that could interfere with or spy on the activity of other users
  • An accounting mechanism that limits the amount of resource units assigned to each user

To ensure safe protection mechanisms, operating systems must use the hardware protection associated with the CPU privileged mode. Otherwise, a user program would be able to directly access the system circuitry and overcome the imposed bounds.
Unix is a multiuser system that enforces the hardware protection of system resources.

Users and Groups

In a multiuser system, each user has a private space on the machine; typically, he owns some quota of the disk space to store files, receives private mail messages, and so on. The operating system must ensure that the private portion of a user space is visible only to its owner. In particular, it must ensure that no user can exploit a system application for the purpose of violating the private space of another user.

All users are identified by a unique number called the User ID (UID). Usually only a restricted number of persons (only they have an account) are allowed to make use of a computer system. When one of these users starts a working session, the system asks for a login name and a password. If the user does not input a valid pair login name-password, the system denies access. Because the password is assumed to be secret, the user's privacy is ensured. Each file in Linux filesystem must be owned by an authorized user.

To selectively share material with other users, each user is a member of one or more user groups , which are identified by a unique number called a user group ID (GUID) . Each file is associated with exactly one (main) group (can also be associated to other-- secondary-- groups ). For example, access can be set so the user owning the file has read and write privileges, the group has read-only privileges, and other users on the system are denied access to the file.

Any Unix-like operating system has a special user called root( or superuser) . The system administrator must log in as root to handle user accounts, perform maintenance tasks such as system backups and program upgrades, and so on. The root user can do almost everything, because the operating system does not apply the usual protection mechanisms to her. In particular, the root user can access every file on the system and can manipulate every running user program.

All operating systems use one fundamental abstraction: the process. A process can be defined either as "an instance of a program in execution" or more formally as the "execution context" of a running program. In traditional operating systems (MS DOS like), a process executes a single sequence of instructions in an address space; the address space is the set of memory addresses that the process is allowed to reference. Modern operating systems (Unix-like, Windows XP etc.) allow processes with multiple execution flows that is, multiple sequences of instructions executed in the same address space.

Multiuser systems must enforce an execution environment in which several processes can be active concurrently and contend for system resources, mainly the CPU (and memory) . Systems that allow concurrent active processes are said to be multiprogramming (or multiprocessing) systems . It is important to distinguish programs from processes; several processes can execute the same program concurrently, while the same process can execute several programs sequentially. Some multiprocessing operating systems are not multiuser; an example is Microsoft Windows 98(but Windows XP Professional, Windows Server and Windows Vista are also multiuser). The easiest way to get a good intuitive feel for a process is to think about multiprogramming systems. Periodically, the operating system decides to stop running one process and start running another, for example, because the first one has had more than its share of CPU time in the past second.
When a process is suspended temporarily like this, it must later be restarted in exactly the same state it had when it was stopped. This means that all information about the process must be explicitly saved somewhere during the suspension. For example, the process may have several files open for reading at once. Associated with each of these files is a pointer giving the current position (i.e., the number of the byte or record to be read next). When a process is temporarily suspended, all these pointers must be saved so that a read call executed after the process is restarted will read the proper data. In many operating systems, all the information about each process, other than the contents of its own address space, is stored in an operating system table called the process table, which is an array (or linked list) of structures, one for each process currently in existence.
Thus, a (suspended) process consists of:
  • its address space, usually called the core image (in honor of the magnetic core memories used in days of yore), and
  • its process table entry, which contains its registers, among other things.
The key process management system calls are those dealing with the creation and termination of processes. Consider a typical example. A process called the command interpreter or shell reads commands from a terminal. The user has just typed a command requesting that a program be compiled. The shell must now create a new process that will run the compiler. When that process has finished the compilation, it executes a system call to terminate itself.
On Windows and other operating systems that have a GUI, (double) clicking on a desktop icon launches a program in much the same way as typing its name at the command prompt. Although we will not discuss GUIs much, they are really simple command interpreters.
If a process can create one or more other processes (usually referred to as child processes) and these processes in turn can create child processes, we quickly arrive at the process tree structure of Fig. 1-5. Related processes that are cooperating to get some job done often need to communicate with one another and synchronize their activities. This communication is called interprocess communication.
Other process system calls are available to request more memory (or release unused memory), wait for a child process to terminate, and overlay its program with a different one.
Occasionally, there is a need to convey information to a running process that is not sitting around waiting for it. For example, a process that is communicating with another process on a different computer does so by sending messages to the remote process over a network. To guard against the possibility that a message or its reply is lost, the sender may request that its own operating system notify it after a specified number of seconds, so that it can retransmit the message if no acknowledgement has been received yet. After setting this timer, the program may continue doing other work.
When the specified number of seconds has elapsed, the operating system sends an alarm signal to the process. The signal causes the process to temporarily suspend whatever it was doing, save its registers on the stack, and start running a special signal handling procedure, for example, to retransmit a presumably lost message. When the signal handler is done, the running process is restarted in the state it was in just before the signal. Signals are the software analog of hardware interrupts. They are generated by a variety of causes in addition to timers expiring. Many traps detected by hardware, such as executing an illegal instruction or using an invalid address, are also converted into signals to the guilty process.
Each person authorized to use a MINIX 3/Linux system is assigned a UID (User IDentification) by the system administrator. Every process started has the UID of the person who started it. A child process has the same UID as its parent. Users can be members of groups, each of which has a GID (Group IDentification).
One UID, called the superuser (in UNIX), has special power and may violate many of the protection rules. In large installations, only the system administrator knows the password needed to become superuser, but many of the ordinary users (especially students) devote considerable effort to trying to find flaws in the system that allow them to become superuser without the password.

On uniprocessor systems, just one process can hold the CPU, and hence just one execution flow can progress at a time. In general, the number of CPUs is always restricted, and therefore only a few processes can progress at once. An operating system component called the scheduler chooses the process that can progress. Some operating systems allow only non preemptable processes, which means that the scheduler is invoked only when a process voluntarily relinquishes the CPU. But processes of a multiuser system must be preemptable; the operating system tracks how long each process holds the CPU and periodically activates the scheduler.

Unix is a multiprocessing operating system with preemptable processes . Even when no user is logged in and no application is running, several system processes monitor the peripheral devices.
In particular, several processes listen at the system terminals waiting for user logins. When a user inputs a login name, the listening process runs a program that validates the user password. If the user identity is acknowledged, the process creates another process that runs a shell into which commands are entered. When a graphical display is activated, one process runs the window manager, and each window on the display is usually run by a separate process. When a user creates a graphics shell, one process runs the graphics windows and a second process runs the shell into which the user can enter the commands(like xterm). For each user command(ls, cp etc.), the shell process creates another process that executes the corresponding program.
Unix-like operating systems adopt a process/kernel model: Each process has the illusion that it's the only process on the machine, and it has exclusive access to the operating system services.
Whenever a process makes a system call (i.e. a request to the kernel), the hardware changes the privilege mode from User Mode to Kernel Mode, and the process starts the execution of a kernel procedure with a strictly limited purpose. In this way, the operating system acts within the execution context of the process in order to satisfy its request. Whenever the request is fully satisfied, the kernel procedure forces the hardware to return to User Mode and the process continues its execution from the instruction following the system call.

Operating System Structure
We will examine five different structures that have been tried, in order to get some idea of the spectrum of possibilities. These are by no means exhaustive, but they give an idea of some designs that have been tried in practice. The five designs are
  • monolithic systems
  • layered systems
  • virtual machines
  • exokernels
  • client-server systems.

Monolithic Systems

By far the most common organization, this approach might well be subtitled "The Big Mess." The structure is that there is no structure. The operating system is written as a collection of procedures, each of which can call any of the other ones whenever it needs to. When this technique is used, each procedure in the system has a well-defined interface in terms of parameters and results, and each one is free to call any other one, if the latter provides some useful computation that the former needs.
To construct the actual object program of the operating system when this approach is used, one first compiles all the individual procedures, or files containing the procedures, and then binds them all together into a single object file using the system linker. In terms of information hiding, there is essentially non every procedure is visible to every other procedure (as opposed to a structure containing modules or packages, in which much of the information is hidden away inside modules, and only the officially designated entry points can be called from outside the module).
Even in monolithic systems, however, it is possible to have at least a little structure. The services (system calls) provided by the operating system are requested by putting the parameters in well-defined places, such as in registers or on the stack, and then executing a special trap instruction known as a kernel call or supervisor call.
This instruction switches the machine from user mode to kernel mode and transfers control to the operating system. (Most CPUs have two modes: kernel mode, for the operating system, in which all instructions are allowed; and user mode, for user programs, in which I/O and certain other instructions are not allowed.)
This is a good time to look at how system calls are performed. Recall that the read call is used like this:

count = read(fd, buffer, nbytes);

Figure 1-16. The 11 steps in making the system call read(fd, buffer, nbytes)
In preparation for calling the read library procedure, which actually makes the read system call, the calling program first pushes the parameters onto the stack, as shown in steps 13 in Fig. 1-16. C and C++ compilers push the parameters onto the stack in reverse order for historical reasons (having to do with making the first parameter to printf, the format string, appear on top of the stack). The first and third parameters are called by value, but the second parameter is passed by reference, meaning that the address of the buffer (indicated by &) is passed, not the contents of the buffer. Then comes the actual call to the library procedure (step 4). This instruction is the normal procedure call instruction used to call all procedures.
The library procedure, possibly written in assembly language, typically puts the system call number in a place where the operating system expects it, such as a register (step 5). Then it executes a trap instruction to switch from user mode to kernel mode and start execution at a fixed address within the kernel (step 6). The kernel code that starts examines the system call number and then dispatches to the correct system call handler, usually via a table of pointers to system call handlers indexed on system call number (step 7). At that point the system call handler runs (step 8). Once the system call handler has completed its work, control may be returned to the user-space library procedure at the instruction following the trap instruction (step 9). This procedure then returns to the user program in the usual way procedure calls return (step 10).
To finish the job, the user program has to clean up the stack, as it does after any procedure call (step 11). Assuming the stack grows downward, as it often does, the compiled code increments the stack pointer exactly enough to remove the parameters pushed before the call to read. The program is now free to do whatever it wants to do next.
In step 9 above, we said "may be returned to the user-space library procedure" for good reason. The system call may block the caller, preventing it from continuing. For example, if it is trying to read from the keyboard and nothing has been typed yet, the caller has to be blocked. In this case, the operating system will look around to see if some other process can be run next. Later, when the desired input is available, this process will get the attention of the system and steps 911 will occur.
This organization suggests a basic structure for the operating system:
  1. A main program that invokes the requested service procedure.
  2. A set of service procedures that carry out the system calls.
  3. A set of utility procedures that help the service procedures.
In this model, for each system call there is one service procedure that takes care of it. The utility procedures do things that are needed by several service procedures, such as fetching data from user programs. This division of the procedures into three layers is shown in Fig. 1-17.

Layered Systems
A generalization of the approach of Fig. 1-17 is to organize the operating system as a hierarchy of layers, each one constructed upon the one below it. The first system constructed in this way was the THE system built at the Technische Hogeschool Eindhoven in the Netherlands by E. W. Dijkstra (1968) and his students. The THE system was a simple batch system for a Dutch computer, the Electrologica X8, which had 32K of 27-bit words (bits were expensive back then).
The system had 6 layers, as shown in Fig. 1-18.
  • Layer 0 dealt with allocation of the processor, switching between processes when interrupts occurred or timers expired. Above layer 0, the system consisted of sequential processes, each of which could be programmed without having to worry about the fact that multiple processes were running on a single processor. In other words, layer 0 provided the basic multiprogramming of the CPU.
  • Layer 1 did the memory management. It allocated space for processes in main memory and on a 512K word drum used for holding parts of processes (pages) for which there was no room in main memory. Above layer 1, processes did not have to worry about whether they were in memory or on the drum; the layer 1 software took care of making sure pages were brought into memory whenever they were needed.
  • Layer 2 handled communication between each process and the operator console. Above this layer each process effectively had its own operator console.
  • Layer 3 took care of managing the I/O devices and buffering the information streams to and from them. Above layer 3 each process could deal with abstract I/O devices with nice properties, instead of real devices with many peculiarities.
  • Layer 4 was where the user programs were found. They did not have to worry about process, memory, console, or I/O management.
  • The system operator process was located in layer 5.
A further generalization of the layering concept was present in the MULTICS system. Instead of layers, MULTICS was organized as a series of concentric rings, with the inner ones being more privileged than the outer ones. When a procedure in an outer ring wanted to call a procedure in an inner ring, it had to make the equivalent of a system call, that is, a TRAP instruction whose parameters were carefully checked for validity before allowing the call to proceed. Although the entire operating system was part of the address space of each user process in MULTICS, the hardware made it possible to designate individual procedures (memory segments, actually) as protected against reading, writing, or executing.
Whereas the THE layering scheme was really only a design aid, because all the parts of the system were ultimately linked together into a single object program, in MULTICS, the ring mechanism was very much present at run time and enforced by the hardware. The advantage of the ring mechanism is that it can easily be extended to structure user subsystems. For example, a professor could write a program to test and grade student programs and run this program in ring n, with the student programs running in ring n + 1 so that they could not change their grades. The Pentium hardware supports the MULTICS ring structure, but no major operating system uses it at present.

Virtual Machines
The initial releases of OS/360 were strictly batch systems. Nevertheless, many 360 users wanted to have timesharing, so various groups, both inside and outside IBM decided to write timesharing systems for it. The official IBM timesharing system, TSS/360, was delivered late, and when it finally arrived it was so big and slow that few sites converted over to it. It was eventually abandoned after its development had consumed some $50 million (Graham, 1970). But a group at IBM's Scientific Center in Cambridge, Massachusetts, produced a radically different system that IBM eventually accepted as a product, and which is now widely used on its mainframes.
This system, originally called CP/CMS and later renamed VM/370 (Seawright and MacKinnon, 1979), was based on a very astute observation: a timesharing system provides
  1. multiprogramming and
  2. an extended machine with a more convenient interface than the bare hardware.
Figure 1-19. The structure of VM/370 with CMS
The essence of VM/370 is to completely separate these two functions. The heart of the system, known as the virtual machine monitor, runs on the bare hardware and does the multiprogramming, providing not one, but several virtual machines to the next layer up, as shown in Fig. 1-19. However, unlike all other operating systems, these virtual machines are not extended machines, with files and other nice features. Instead, they are exact copies of the bare hardware, including kernel/user mode, I/O, interrupts, and everything else the real machine has.
Because each virtual machine is identical to the true hardware, each one can run any operating system that will run directly on the bare hardware. Different virtual machines can, and frequently do, run different operating systems. Some run one of the descendants of OS/360 for batch or transaction processing, while others run a single-user, interactive system called CMS (Conversational Monitor System) for timesharing users.
When a CMS program executes a system call, the call is trapped to the operating-system in its own virtual machine, not to VM/370, just as it would if it were running on a real machine instead of a virtual one. CMS then issues the normal hardware I/O instructions for reading its virtual disk or whatever is needed to carry out the call. These I/O instructions are trapped by VM/370, which then performs them as part of its simulation of the real hardware. By making a complete separation of the functions of multiprogramming and providing an extended machine, each of the pieces can be much simpler, more flexible, and easier to maintain.
The idea of a virtual machine is used nowadays in a different context: running old MS-DOS programs on a Pentium. When designing the Pentium and its software, both Intel and Microsoft realized that there would be a big demand for running old software on new hardware. For this reason, Intel provided a virtual 8086 mode on the Pentium. In this mode, the machine acts like an 8086 (which is identical to an 8088 from a software point of view), including 16-bit addressing with a 1-MB limit.
This mode is used by Windows, and other operating systems for running old MS-DOS programs. These programs are started up in virtual 8086 mode. As long as they execute normal instructions, they run on the bare hardware. However, when a program tries to trap to the operating system to make a system call, or tries to do protected I/O directly, a trap to the virtual machine monitor occurs.
Two variants on this design are possible. In the first one, MS-DOS itself is loaded into the virtual 8086's address space, so the virtual machine monitor just reflects the trap back to MS-DOS, just as would happen on a real 8086. When MS-DOS later tries to do the I/O itself, that operation is caught and carried out by the virtual machine monitor.
In the other variant, the virtual machine monitor just catches the first trap and does the I/O itself, since it knows what all the MS-DOS system calls are and thus knows what each trap is supposed to do. This variant is less pure than the first one, since it emulates only MS-DOS correctly, and not other operating systems, as the first one does. On the other hand, it is much faster, since it saves the trouble of starting up MS-DOS to do the I/O. A further disadvantage of actually running MS-DOS in virtual 8086 mode is that MS-DOS fiddles around with the interrupt enable/disable bit quite a lot, all of which must be emulated at considerable cost.
It is worth noting that neither of these approaches are really the same as VM/370, since the machine being emulated is not a full Pentium, but only an 8086. With the VM/370 system, it is possible to run VM/370, itself, in the virtual machine. Even the earliest versions of Windows require at least a 286 and cannot be run on a virtual 8086.
Several virtual machine implementations are marketed commercially. For companies that provide web-hosting services, it can be more economical to run multiple virtual machines on a single fast server (perhaps one with multiple CPUs) than to run many small computers, each hosting a single Web site. VMWare and Microsoft's Virtual PC are marketed for such installations. These programs use large files on a host system as simulated disks for their guest systems. To achieve efficiency they analyze guest system program binaries and allow safe code to run directly on the host hardware, trapping instructions that make operating system calls. Such systems are also useful in education. For instance, students working on MINIX 3 lab assignments can work using MINIX 3 as a guest operating system on VMWare on a Windows, Linux or UNIX host with no risk of damaging other software installed on the same PC. Most professors teaching other subjects would be very nervous about sharing laboratory computers with an operating systems course where student mistakes could corrupt or erase disk data.
Another are a where virtual machines are used, but in a somewhat different way, is for running Java programs. When Sun Microsystems invented the Java programming language, it also invented a virtual machine (i.e., a computer architecture) called the JVM (Java Virtual Machine). The Java compiler produces code for JVM, which then typically is executed by a software JVM interpreter(nowadays JIT compiler). The advantage of this approach is that the JVM code can be shipped over the Internet to any computer that has a JVM interpreter and run there. If the compiler had produced SPARC or Pentium binary programs, for example, they could not have been shipped and run anywhere as easily. (Of course, Sun could have produced a compiler that produced SPARC binaries and then distributed a SPARC interpreter, but JVM is a much simpler architecture to interpret.) Another advantage of using JVM is that if the interpreter is implemented properly, which is not completely trivial, incoming JVM programs can be checked for safety and then executed in a protected environment so they cannot steal data or do any damage.

With VM/370, each user process gets an exact copy of the actual computer. With virtual 8086 mode on the Pentium, each user process gets an exact copy of a different computer. Going one step further, researchers at M.I.T. built a system that gives each user a clone of the actual computer, but with a subset of the resources (Engler et al., 1995; and Leschke, 2004). Thus one virtual machine might get disk blocks 0 to 1023, the next one might get blocks 1024 to 2047, and so on.
At the bottom layer, running in kernel mode, is a program called the exokernel. Its job is to allocate resources to virtual machines and then check attempts to use them to make sure no machine is trying to use somebody else's resources. Each user-level virtual machine can run its own operating system, as on VM/370 and the Pentium virtual 8086s, except that each one is restricted to using only the resources it has asked for and been allocated.
The advantage of the exokernel scheme is that it saves a layer of mapping. In the other designs, each virtual machine thinks it has its own disk, with blocks running from 0 to some maximum, so the virtual machine monitor must maintain tables to remap disk addresses (and all other resources). With the exokernel, this remapping is not needed. The exokernel need only keep track of which virtual machine has been assigned which resource. This method still has the advantage of separating the multiprogramming (in the exokernel) from the user operating system code (in user space), but with less overhead, since all the exokernel has to do is keep the virtual machines out of each other's hair.

Client-Server Model
VM/370 gains much in simplicity by moving a large part of the traditional operating system code (implementing the extended machine) into a higher layer, CMS. Nevertheless, VM/370 itself is still a complex program because simulating a number of virtual 370s is not that simple (especially if you want to do it reasonably efficiently).
A trend in modern operating systems is to take this idea of moving code up into higher layers even further and remove as much as possible from the operating system, leaving a minimal kernel. The usual approach is to implement most of the operating system functions in user processes. To request a service, such as reading a block of a file, a user process (now known as the client process) sends the request to a server process, which then does the work and sends back the answer.
Figure 1-20. The client-server model

In this model, shown in Fig. 1-20, all the kernel does is handle the communication between clients and servers. By splitting the operating system up into parts, each of which only handles one facet of the system, such as file service, process service, terminal service, or memory service, each part becomes small and manageable. Furthermore, because all the servers run as user-mode processes, and not in kernel mode, they do not have direct access to the hardware. As a consequence, if a bug in the file server is triggered, the file service may crash, but this will not usually bring the whole machine down.

Figure 1-21. The client-server model in a distributed system
Another advantage of the client-server model is its adaptability to use in distributed systems (see Fig. 1-21). If a client communicates with a server by sending it messages, the client need not know whether the message is handled locally in its own machine, or whether it was sent across a network to a server on a remote machine. As far as the client is concerned, the same thing happens in both cases: a request was sent and a reply came back. The picture painted above of a kernel that handles only the transport of messages from clients to servers and back is not completely realistic. Some operating system functions (such as loading commands into the physical I/O device registers) are difficult, if not impossible, to do from user-space programs. There are two ways of dealing with this problem.
  • One way is to have some critical server processes (e.g., I/O device drivers) actually run in kernel mode, with complete access to all the hardware, but still communicate with other processes using the normal message mechanism. A variant of this mechanism was used in earlier versions of MINIX where drivers were compiled into the kernel but ran as separate processes.
  • The other way is to build a minimal amount of mechanism into the kernel but leave the policy decisions up to servers in user space. For example, the kernel might recognize that a message sent to a certain special address means to take the contents of that message and load it into the I/O device registers for some disk, to start a disk read. In this example, the kernel would not even inspect the bytes in the message to see if they were valid or meaningful; it would just blindly copy them into the disk's device registers. (Obviously, some scheme for limiting such messages to authorized processes only must be used.) This is how MINIX 3 works, drivers are in user space and use special kernel calls to request reads and writes of I/O registers or to access kernel information. The split between mechanism and policy is an important concept; it occurs again and again in operating systems in various contexts.

Linux Kernel Architecture
As stated before, most Unix kernels are monolithic: each kernel layer is integrated into the whole kernel program and runs in Kernel Mode on behalf of the current process. In contrast, microkernel operating systems demand a very small set of functions from the kernel, generally including:
  • a few synchronization primitives
  • a simple scheduler, and
  • an interprocess communication mechanism
Several system processes that run on top of the microkernel implement other operating system- layer functions, like memory allocators, device drivers, and system call handlers.

Although academic research on operating systems is oriented toward microkernels , such operating systems are generally slower than monolithic ones, because the explicit message passing between the different layers of the operating system has a cost. However, microkernel operating systems might have some theoretical advantages over monolithic ones:
  • Microkernels force the system programmers to adopt a modularized approach, because each operating system layer is a relatively independent program that must interact with the other layers through well-defined and clean software interfaces.
  • Moreover, an existing microkernel operating system can be easily ported to other architectures fairly easily, because all hardware-dependent components are generally encapsulated in the microkernel code.
  • Finally, microkernel operating systems tend to make better use of random access memory (RAM) than monolithic ones, because system processes that aren't implementing needed functionalities might be swapped out or destroyed.
To achieve many of the theoretical advantages of microkernels without introducing performance penalties, the Linux kernel offers modules . A module is an object file whose code can be linked to (and unlinked from) the kernel at runtime. The object code usually consists of a set of functions that implements a filesystem, a device driver, or other features at the kernel's upper layer. The module, unlike the external layers of microkernel operating systems, does not run as a specific process. Instead, it is executed in Kernel Mode on behalf of the current process, like any other statically linked kernel function.

The main advantages of using modules include:
  • modularized approach Because any module can be linked and unlinked at runtime, system programmers must introduce well-defined software interfaces to access the data structures handled by modules. This makes it easy to develop new modules.
  • Platform independence Even if it may rely on some specific hardware features, a module doesn't depend on a fixed hardware platform. For example, a disk driver module that relies on the SCSI standard works as well on an IBM-compatible PC as it does on Hewlett-Packard's Alpha.
  • Frugal main memory usage A module can be linked to the running kernel when its functionality is required and unlinked when it is no longer useful; this is quite useful for small embedded systems(in general on limited resources).
  • No performance penalty Once linked in, the object code of a module is equivalent to the object code of the statically linked kernel. Therefore, no explicit message passing is required when the functions of the module are invoked.
    A small performance penalty occurs when the module is linked and unlinked. However, this penalty can be compared to the penalty caused by the creation and deletion of system processes in microkernel operating systems.

System Calls
Armed with our general knowledge of how MINIX 3 deals with processes and files, we can now begin to look at the interface between the operating system and its application programs, that is, the set of system calls. Although this discussion specifically refers to POSIX (International Standard 9945-1), hence also to MINI 3, UNIX, and Linux, most other modern operating systems have system calls that perform the same functions, even if the details differ. Since the actual mechanics of issuing a system call are highly machine dependent, and often must be expressed in assembly code, a procedure library is provided to make it possible to make system calls from C programs.
It is useful to keep the following in mind: any single-CPU computer can execute only one instruction at a time. If a process is running a user program in user mode and needs a system service, such as reading data from a file, it has to execute a trap or system call instruction to transfer control to the operating system. The operating system then figures out what the calling process wants by inspecting the parameters. Then it carries out the system call and returns control to the instruction following the system call. In a sense, making a system call is like making a special kind of procedure call, only system calls enter the kernel or other privileged operating system components and procedure calls do not.
To make the system call mechanism clearer, let us take a quick look at read. It has three parameters: the first one specifying the file, the second one specifying the buffer, and the third one specifying the number of bytes to read. A call to read from a C program might look like this:

count = read(fd, buffer, nbytes);

The system call (and the library procedure) return the number of bytes actually read in count. This value is normally the same as nbytes, but may be smaller, if, for example, end-of-file is encountered while reading.
If the system call cannot be carried out, either due to an invalid parameter or a disk error, count is set to 1, and the error number is put in a global variable, errno. Programs should always check the results of a system call to see if an error occurred.
MINIX 3 has a total of 53 main system calls. These are listed in Fig. 1-9, grouped for convenience in six categories. A few other calls exist, but they have very specialized uses so we will omit them here. In the following sections we will briefly examine each of the calls of Fig. 1-9 to see what it does. To a large extent, the services offered by these calls determine most of what the operating system has to do, since the resource management on personal computers is minimal (at least compared to big machines with many users).

Figure 1-9. The main MINIX system calls. fd is a file descriptor; n is a byte count

This is a good place to point out that the mapping of POSIX procedure calls onto system calls is not necessarily one-to-one. The POSIX standard specifies a number of procedures that a conformant system must supply, but it does not specify whether they are system calls, library calls, or something else. In some cases, the POSIX procedures are supported as library routines in MINIX 3. In others, several required procedures are only minor variations of one another, and one system call handles all of them.

System Calls for Process Management
The first group of calls in Fig. 1-9 deals with process management. Fork is a good place to start the discussion. Fork is the only way to create a new process in MINIX 3. It creates an exact duplicate of the original process, including all the file descriptors, registers everything. After the fork, the original process and the copy (the parent and child) go their separate ways. All the variables have identical values at the time of the fork, but since the parent's data are copied to create the child, subsequent changes in one of them do not affect the other one. (The program text, which is unchangeable, is shared between parent and child.) The fork call returns a value, which is zero in the child and equal to the child's process identifier(PID) in the parent. Using the returned PID, the two processes can see which one is the parent process and which one is the child process.
In most cases, after a fork, the child will need to execute different code from the parent. Consider the shell. It reads a command from the terminal, forks off a child process, waits for the child to execute the command, and then reads the next command when the child terminates. To wait for the child to finish, the parent executes a waitpid system call, which just waits until the child terminates (any child if more than one exists). Waitpid can wait for a specific child, or for any old child by setting the first parameter to 1. When waitpid completes, the address pointed to by the second parameter, statloc, will be set to the child's exit status (normal or abnormal termination and exit value). Various options are also provided, specified by the third parameter. The waitpid call replaces the previous wait call, which is now obsolete but is provided for reasons of backward compatibility.
Now consider how fork is used by the shell. When a command is typed, the shell forks off a new process. This child process must execute the user command. It does this by using the execve system call, which causes its entire core image to be replaced by the file named in its first parameter. (Actually, the system call itself is exec, but several different library procedures call it with different parameters and slightly different names. We will treat these as system calls here.) A highly simplified shell illustrating the use of fork, waitpid, and execve is shown in Fig. 1-10.

Figure 1-10. A stripped-down shell. Throughout this book, TRUE is assumed to be defined as 1
#define TRUE 1

while (TRUE){ /* repeat forever */
type_prompt(); /* display prompt on the screen */
read_command(command, parameters); /* read input from terminal */
if (fork() != 0){ /* fork off child process */
/* Parent code. */
waitpid(1, &status, 0); /* wait for child to exit */
} else {
/* Child code. */
execve(command, parameters, 0); /* execute command */

In the most general case, execve has three parameters:
  • the name of the file to be executed
  • a pointer to the argument array and
  • a pointer to the environment array.
These will be described shortly. Various library routines, including execl, execv, execle, and execve, are provided to allow the parameters to be omitted or specified in various ways. Throughout this text we will use the name exec to represent the system call invoked by all of these.
Let us consider the case of a command such as

$ cp file1 file2 

used to copy file1 to file2. After the shell has forked, the child process locates and executes the file cp and passes to it the names of the source and target files.
The main program of cp (and main program of most other C programs) contains the declaration

main(argc, argv, envp)

where argc is a count of the number of items on the command line, including the program name. For the example above, argc is 3.
The second parameter, argv, is a pointer to an array of strings. Element i of that array is a pointer to the i-th string on the command line. In our example, argv[0] would point to the string "cp", argv[1] would point to the string "file1", and argv[2] would point to the string "file2".
The third parameter of main, envp, is a pointer to the environment, an array of strings containing assignments of the form name=value used to pass information such as the terminal type and home directory name to a program. In Fig. 1-10, no environment is passed to the child, so the third parameter of execve is a zero.
If exec seems complicated, do not despair; it is (semantically) the most complex of all the POSIX system calls. All the other ones are much simpler. As an example of a simple one, consider exit, which processes should use when they are finished executing. It has one parameter, the exit status (0 to 255), which is returned to the parent via statloc in the waitpid system call. The low-order byte of status contains the termination status, with 0 being normal termination and the other values being various error conditions. The high-order byte contains the child's exit status (0 to 255). For example, if a parent process executes the statement

n = waitpid(1, &statloc, options);

it will be suspended until some child process terminates. If the child exits with, say, 4 as the parameter to exit, the parent will be awakened with n set to the child's PID and statloc set to 0x0400 (the C convention of prefixing hexadecimal constants with 0x will be used throughout this text).
Processes in MINIX 3 have their memory divided up into three segments:
  • the text segment (i.e., the program code),
  • the data segment (i.e., the variables), and
  • the stack segment.
The data segment grows upward and the stack grows downward, as shown in Fig. 1-11. Between them is a gap of unused address space. The stack grows into the gap automatically, as needed, but expansion of the data segment is done explicitly by using a system call, brk, which specifies the new address where the data segment is to end. This address may be more than the current value (data segment is growing) or less than the current value (data segment is shrinking). The parameter must, of course, be less than the stack pointer or the data and stack segments would overlap, which is forbidden.
As a convenience for programmers, a library routine sbrk is provided that also changes the size of the data segment, only its parameter is the number of bytes to add to the data segment (negative parameters make the data segment smaller). It works by keeping track of the current size of the data segment, which is the value returned by brk, computing the new size, and making a call asking for that number of bytes. The brk and sbrk calls, however, are not defined by the POSIX standard. Programmers are encouraged to use the malloc library procedure for dynamically allocating storage, and the underlying implementation of malloc was not thought to be a suitable subject for standardization since few programmers use it directly.

The next process system call is also the simplest, getpid. It just returns the caller's PID. Remember that in fork, only the parent was given the child's PID. If the child wants to find out its own PID, it must use getpid. The getpgrp call returns the PID of the caller's process group. setsid creates a new session and sets the process group's PID to the caller's. Sessions are related to an optional feature of POSIX, job control, which is not supported by MINIX 3 and which will not concern us further.

The last process management system call, ptrace, is used by debugging programs to control the program being debugged. It allows the debugger to read and write the controlled process' memory and manage it in other ways.

System Calls for Signaling
Although most forms of interprocess communication are planned, situations exist in which unexpected communication is needed. For example:
  • if a user accidently tells a text editor to list the entire contents of a very long file, and then realizes the error, some way is needed to interrupt the editor. In MINIX 3, the user can hit the CTRL-C key on the keyboard, which sends a signal to the editor. The editor catches the signal and stops the print-out.
  • Signals can also be used to report certain traps detected by the hardware, such as illegal instruction or floating point overflow.
  • Timeouts are also implemented as signals.
When a signal is sent to a process that has not announced its willingness to accept that signal, the process is simply killed without further ado. To avoid this fate, a process can use the sigaction system call to announce that it is prepared to accept some signal type, and to provide the address of the signal handling procedure and a place to store the address of the current one.
  1. After a sigaction call, if a signal of the relevant type is generated (e.g., by pressing CTRL-C),
  2. the state of the process is pushed onto its own stack, and
  3. then the signal handler is called. It may run for as long as it wants to and perform any system calls it wants to. In practice, though, signal handlers are usually fairly short. When the signal handling procedure is done,
  4. it calls sigreturn to continue where it left off before the signal.
    The sigaction call replaces the older signal call, which is now provided as a library procedure, however, for backward compatibility.
Signals can be blocked in MINIX 3. A blocked signal is held pending until it is unblocked. It is not delivered, but also not lost. The sigprocmask call allows a process to define the set of blocked signals by presenting the kernel with a bitmap. It is also possible for a process to ask for the set of signals currently pending but not allowed to be delivered due to their being blocked. The sigpending call returns this set as a bitmap. Finally, the sigsuspend call allows a process to atomically set the bitmap of blocked signals and suspend itself.

Instead of providing a function to catch a signal, the program may also specify the constant SIG_IGN to have all subsequent signals of the specified type ignored, or SIG_DFL to restore the default action of the signal when it occurs. The default action is either to kill the process or ignore the signal, depending upon the signal. As an example of how SIG_IGN is used, consider what happens when the shell forks off a background process as a result of

$ command & 

It would be undesirable for a SIGINT signal (generated by pressing CTRL-C) to affect the background process, so after the fork but before the exec, the shell does

sigaction(SIGINT, SIG_IGN, NULL);


sigaction(SIGQUIT, SIG_IGN, NULL);

to disable the SIGINT and SIGQUIT signals. (SIGQUIT is generated by CTRL-\; it is the same as SIGINT generated by CTRL-C except that if it is not caught or ignored it makes a core dump of the process killed.) For foreground processes (no ampersand), these signals are not ignored.

Hitting CTRL-C is not the only way to send a signal. The kill system call allows a process to signal another process (provided they have the same UID; unrelated processes cannot signal each other). Getting back to the example of background processes used above, suppose a background process is started up, but later it is decided that the process should be terminated. SIGINT and SIGQUIT have been disabled, so something else is needed. The solution is to use the kill program, which uses the kill system call to send a signal to any process. By sending signal 9 (SIGKILL), to a background process, that process can be killed. SIGKILL cannot be caught or ignored.
For many real-time applications, a process needs to be interrupted after a specific time interval to do something, such as to retransmit a potentially lost packet over an unreliable communication line. To handle this situation, the alarm system call has been provided. The parameter specifies an interval, in seconds, after which a SIGALRM signal is sent to the process. A process may only have one alarm outstanding at any instant. If an alarm call is made with a parameter of 10 seconds, and then 3 seconds later another alarm call is made with a parameter of 20 seconds, only one signal will be generated, 20 seconds after the second call. The first signal is canceled by the second call to alarm. If the parameter to alarm is zero, any pending alarm signal is canceled.
If an alarm signal is not caught, the default action is taken and the signaled process is killed.

It sometimes occurs that a process has nothing to do until a signal arrives. For example, consider a computer-aided-instruction program that is testing reading speed and comprehension. It displays some text on the screen and then calls alarm to signal it after 30 seconds. While the student is reading the text, the program has nothing to do. It could sit in a tight loop doing nothing, but that would waste CPU time that another process or user might need. A better idea is to use pause, which tells MINIX 3 to suspend the process until the next signal.

System Calls for File Management
Many system calls relate to the file system. In this section we will look at calls that operate on individual files; in the next one we will examine those that involve directories or the file system as a whole. To create a new file, the creat call is used (why the call is creat and not create has been lost in the mists of time). Its parameters provide the name of the file and the protection mode. Thus

fd = creat("abc", 0751);

creates a file called abc with mode 0751 octal (in C, a leading zero means that a constant is in octal). The low-order 9 bits of 0751 specify the rwx bits for the owner (7 means read-write-execute permission), his group (5 means read-execute), and others (1 means execute only).
Creat not only creates a new file but also opens it for writing, regardless of the file's mode. The file descriptor returned, fd, can be used to write the file. If a creat is done on an existing file, that file is truncated to length 0, provided, of course, that the permissions are all right.
The creat call is obsolete, as open can now create new files, but it has been included for backward compatibility.
Special files are created using mknod rather than creat. A typical call is

fd = mknod("/dev/ttyc2", 020744, 0x0402);

which creates a file named /dev/ttyc2 (the usual name for console 2) and gives it mode 020744 octal (a character special file with protection bits rwxr--r--). The third parameter contains the major device (4) in the high-order byte and the minor device (2) in the low-order byte. The major device could have been anything, but a file named /dev/ttyc2 ought to be minor device 2. Calls to mknod fail unless the caller is the superuser.
To read or write an existing file, the file must first be opened using open. This call specifies the file name to be opened, either as an absolute path name or relative to the working directory, and a code of O_RDONLY, O_WRONLY, or O_RDWR, meaning open for reading, writing, or both. The file descriptor returned can then be used for reading or writing. Afterward, the file can be closed by close, which makes the file descriptor available for reuse on a subsequent creat or open.
The most heavily used calls are undoubtedly read and write. We saw read earlier; write has the same parameters.
Although most programs read and write files sequentially, for some applications programs need to be able to access any part of a file at random. Associated with each file is a pointer that indicates the current position in the file. When reading (writing) sequentially, it normally points to the next byte to be read (written). The lseek call changes the value of the position pointer, so that subsequent calls to read or write can begin anywhere in the file, or even beyond the end.
lseek has three parameters: the first is the file descriptor for the file, the second is a file position, and the third tells whether the file position is relative to the beginning of the file, the current position, or the end of the file. The value returned by lseek is the absolute position in the file after changing the pointer.
For each file, MINIX 3 keeps track of the file mode (regular file, special file, directory, and so on), size, time of last modification, and other information. Programs can ask to see this information via the stat and fstat system calls. These differ only in that the former specifies the file by name, whereas the latter takes a file descriptor, making it useful for open files, especially standard input and standard output, whose names may not be known. Both calls provide as the second parameter a pointer to a structure where the information is to be put. The structure is shown in Fig. 1-12.

Figure 1-12. The structure used to return information for the stat and fstat system calls. In the actual code, symbolic names are used for some of the types.

When manipulating file descriptors, the dup call is occasionally helpful. Consider, for example, a program that needs to close standard output (file descriptor 1), substitute another file as standard output, call a function that writes some output onto standard output, and then restore the original situation. Just closing file descriptor 1 and then opening a new file will make the new file standard output (assuming standard input, file descriptor 0, is in use), but it will be impossible to restore the original situation later. The solution is first to execute the statement

fd = dup(1);

which uses the dup system call to allocate a new file descriptor, fd, and arrange for it to correspond to the same file as standard output. Then standard output can be closed and a new file opened and used. When it is time to restore the original situation, file descriptor 1 can be closed, and then

n = dup(fd);

executed to assign the lowest file descriptor, namely, 1, to the same file as fd. Finally, fd can be closed and we are back where we started.

The dup call has a variant that allows an arbitrary unassigned file descriptor to be made to refer to a given open file. It is called by

dup2(fd, fd2);

where fd refers to an open file and fd2 is the unassigned file descriptor that is to be made to refer to the same file as fd. Thus if fd refers to standard input (file descriptor 0) and fd2 is 4, after the call, file descriptors 0 and 4 will both refer to standard input.

Interprocess communication in MINIX 3 uses pipes, as described earlier. When a user types

$ cat file1 file2 | sort 

the shell creates a pipe and arranges for standard output of the first process to write to the pipe, so standard input of the second process can read from it. The pipe system call creates a pipe and returns two file descriptors, one for writing and one for reading. The call is

where fd is an array of two integers and fd[0] is the file descriptor for reading and fd[1] is the one for writing. Typically, a fork comes next, and the parent closes the file descriptor for reading and the child closes the file descriptor for writing (or vice versa), so when they are done, one process can read the pipe and the other can write on it.

Figure 1-13 depicts a skeleton procedure that creates two processes, with the output of the first one piped into the second one. (A more realistic example would do error checking and handle arguments.) First a pipe is created, and then the procedure forks, with the parent eventually becoming the first process in the pipeline and the child process becoming the second one. Since the files to be executed, process1 and process2, do not know that they are part of a pipeline, it is essential that the file descriptors be manipulated so that the first process' standard output be the pipe and the second one's standard input be the pipe. The parent first closes off the file descriptor for reading from the pipe. Then it closes standard output and does a DUP call that allows file descriptor 1 to write on the pipe. It is important to realize that dup always returns the lowest available file descriptor, in this case, 1. Then the program closes the other pipe file descriptor.

Figure 1-13. A skeleton for setting up a two-process pipeline

After the exec call, the process started will have file descriptors 0 and 2 be unchanged, and file descriptor 1 for writing on the pipe. The child code is analogous. The parameter to execl is repeated because the first one is the file to be executed and the second one is the first parameter, which most programs expect to be the file name.
The next system call, ioctl, is potentially applicable to all special files. It is, for instance, used by block device drivers like the SCSI driver to control tape and CD-ROM devices. Its main use, however, is with special character files, primarily terminals. POSIX defines a number of functions which the library translates into ioctl calls. The tcgetattr and tcsetattr library functions use ioctl to change the characters used for correcting typing errors on the terminal, changing the terminal mode, and so forth.
Traditionally, there are three terminal modes, cooked, raw, and cbreak. Cooked mode is the normal terminal mode, in which the erase and kill characters work normally, CTRL-S and CTRL-Q can be used for stopping and starting terminal output, CTRL-D means end of file, CTRL-C generates an interrupt signal, and CTRL-\ generates a quit signal to force a core dump.
In raw mode, all of these functions are disabled; consequently, every character is passed directly to the program with no special processing. Furthermore, in raw mode, a read from the terminal will give the program any characters that have been typed, even a partial line, rather than waiting for a complete line to be typed, as in cooked mode. Screen editors often use this mode.

Cbreak mode is in between. The erase and kill characters for editing are disabled, as is CTRL-D, but CTRL-S, CTRL-Q, CTRL-C, and CTRL-\ are enabled. Like raw mode, partial lines can be returned to programs (if intraline editing is turned off there is no need to wait until a whole line has been receivedthe user cannot change his mind and delete it, as he can in cooked mode).
POSIX does not use the terms cooked, raw, and cbreak. In POSIX terminology canonical mode corresponds to cooked mode. In this mode there are eleven special characters defined, and input is by lines. In noncanonical mode a minimum number of characters to accept and a time, specified in units of 1/10th of a second, determine how a read will be satisfied. Under POSIX there is a great deal of flexibility, and various flags can be set to make noncanonical mode behave like either cbreak or raw mode. The older terms are more descriptive, and we will continue to use them informally.
Ioctl has three parameters, for example a call to tcsetattr to set terminal parameters will result in

ioctl(fd, TCSETS, &termios);

The first parameter specifies a file, the second one specifies an operation, and the third one is the address of the POSIX structure that contains flags and the array of control characters. Other operation codes instruct the system to postpone the changes until all output has been sent, cause unread input to be discarded, and return the current values.

The access system call is used to determine whether a certain file access is permitted by the protection system. It is needed because some programs can run using a different user's UID. This SETUID mechanism will be described later.
The rename system call is used to give a file a new name. The parameters specify the old and new names.
Finally, the fcntl call is used to control files, somewhat analogous to ioctl (i.e., both of them are horrible hacks). It has several options, the most important of which is for advisory file locking. Using fcntl, it is possible for a process to lock and unlock parts of files and test part of a file to see if it is locked. The call does not enforce any lock semantics. Programs must do this themselves.

System Calls for Directory Management
In this section we will look at some system calls that relate more to directories or the file system as a whole, rather than just to one specific file as in the previous section. The first two calls, mkdir and rmdir, create and remove empty directories, respectively. The next call is link. Its purpose is to allow the same file to appear under two or more names, often in different directories. A typical use is to allow several members of the same programming team to share a common file, with each of them having the file appear in his own directory, possibly under different names. Sharing a file is not the same as giving every team member a private copy, because having a shared file means that changes that any member of the team makes are instantly visible to the other membersthere is only one file. When copies are made of a file, subsequent changes made to one copy do not affect the other ones.
To see how link works, consider the situation of Fig. 1-14(a). Here are two users, ast and jim, each having their own directories with some files. If ast now executes a program containing the system call

link("/usr/jim/memo", "/usr/ast/note");

the file memo in jim's directory is now entered into ast's directory under the name note. Thereafter, /usr/jim/memo and /usr/ast/note refer to the same file.

Figure 1-14. (a) Two directories before linking /usr/jim/memo to ast's directory. (b) The same directories after linking.

Understanding how link works will probably make it clearer what it does. Every file in UNIX has a unique number, its i-number, that identifies it. This inumber is an index into a table of i-nodes, one per file, telling who owns the file, where its disk blocks are, and so on. A directory is simply a file containing a set of (i-number, ASCII name) pairs. In the first versions of UNIX, each directory entry was 16 bytes2 bytes for the i-number and 14 bytes for the name. A more complicated structure is needed to support long file names, but conceptually a directory is still a set of (i-number, ASCII name) pairs. In Fig. 1-14, mail has inumber 16, and so on. What link does is simply create a new directory entry with a (possibly new) name, using the i-number of an existing file. In Fig. 1-14(b), two entries have the same i-number (70) and thus refer to the same file. If either one is later removed, using the unlink system call, the other one remains. If both are removed, UNIX sees that no entries to the file exist (a field in the i-node keeps track of the number of directory entries pointing to the file), so the file is removed from the disk.

As we have mentioned earlier, the mount system call allows two file systems to be merged into one. A common situation is to have the root file system containing the binary (executable) versions of the common commands and other heavily used files, on a hard disk. The user can then insert a CD-ROM with files to be read into the CD-ROM drive.
By executing the mount system call, the CD-ROM file system can be attached to the root file system, as shown in Fig. 1-15. A typical statement in C to perform the mount is

mount("/dev/cdrom0", "/mnt", 0);

where the first parameter is the name of a block special file for CD-ROM drive 0, the second parameter is the place in the tree where it is to be mounted, and the third one tells whether the file system is to be mounted read-write or read-only.

Figure 1-15. (a) File system before the mount. (b) File system after the mount

After the mount call, a file on CD-ROM drive 0 can be accessed by just using its path from the root directory or the working directory, without regard to which drive it is on. In fact, second, third, and fourth drives can also be mounted anywhere in the tree. The mount call makes it possible to integrate removable media into a single integrated file hierarchy, without having to worry about which device a file is on. Although this example involves CD-ROMs, hard disks or portions of hard disks (often called partitions or minor devices) can also be mounted this way. When a file system is no longer needed, it can be unmounted with the umount system call.

MINIX 3 maintains a block cache cache of recently used blocks in main memory to avoid having to read them from the disk if they are used again quickly. If a block in the cache is modified (by a write on a file) and the system crashes before the modified block is written out to disk, the file system will be damaged. To limit the potential damage, it is important to flush the cache periodically, so that the amount of data lost by a crash will be small. The system call sync tells MINIX 3 to write out all the cache blocks that have been modified since being read in. When MINIX 3 is started up, a program called update is started as a background process to do a sync every 30 seconds, to keep flushing the cache.
Two other calls that relate to directories are chdir and chroot. The former changes the working directory and the latter changes the root directory. After the call


an open on the file xyz will open /usr/ast/test/xyz. chroot works in an analogous way. Once a process has told the system to change its root directory, all absolute path names (path names beginning with a "/") will start at the new root. Why would you want to do that? For securityserver programs for protocols such as FTP (File Transfer Protocol) and HTTP (HyperText Transfer Protocol) do this so remote users of these services can access only the portions of a file system below the new root. Only superusers may execute chroot, and even superusers do not do it very often.

System Calls for Protection
In MINIX 3 every file has an 11-bit mode used for protection. Nine of these bits are the read-write-execute bits for the owner, group, and others. The chmod system call makes it possible to change the mode of a file. For example, to make a file read-only by everyone except the owner, one could execute

chmod("file", 0644);

The other two protection bits, 02000 and 04000, are the SETGID (set-group-id) and SETUID (set-user-id) bits, respectively. When any user executes a program with the SETUID bit on, for the duration of that process the user's effective UID is changed to that of the file's owner. This feature is heavily used to allow users to execute programs that perform superuser only functions, such as creating directories. Creating a directory uses mknod, which is for the superuser only. By arranging for the mkdir program to be owned by the superuser and have mode 04755, ordinary users can be given the power to execute mknod but in a highly restricted way.

When a process executes a file that has the SETUID or SETGID bit on in its mode, it acquires an effective UID or GID different from its real UID or GID. It is sometimes important for a process to find out what its real and effective UID or GID is. The system calls getuid and getgid have been provided to supply this information. Each call returns both the real and effective UID or GID, so four library routines are needed to extract the proper information: getuid, getgid, geteuid, and getegid. The first two get the real UID/GID, and the last two the effective ones.
Ordinary users cannot change their UID, except by executing programs with the SETUID bit on, but the superuser has another possibility: the setuid system call, which sets both the effective and real UIDs. setgid sets both GIDs. The superuser can also change the owner of a file with the chown system call. In short, the superuser has plenty of opportunity for violating all the protection rules, which explains why so many students devote so much of their time to trying to become superuser.
The last two system calls in this category can be executed by ordinary user processes. The first one, umask, sets an internal bit mask within the system, which is used to mask off mode bits when a file is created. After the call


the mode supplied by creat and mknod will have the 022 bits masked off before being used. Thus the call

creat("file", 0777);

will set the mode to 0755 rather than 0777. Since the bit mask is inherited by child processes, if the shell does a umask just after login, none of the user's processes in that session will accidently create files that other people can write on.
When a program owned by the root has the SETUID bit on, it can access any file, because its effective UID is the superuser. Frequently it is useful for the program to know if the person who called the program has permission to access a given file. If the program just tries the access, it will always succeed, and thus learn nothing.
What is needed is a way to see if the access is permitted for the real UID. The access system call provides a way to find out. The mode parameter is 4 to check for read access, 2 for write access, and 1 for execute access. Combinations of these values are also allowed. For example, with mode equal to 6, the call returns 0 if both read and write access are allowed for the real ID; otherwise1 is returned. With mode equal to 0, a check is made to see if the file exists and the directories leading up to it can be searched.

Although the protection mechanisms of all UNIX-like operating systems are generally similar, there are some differences and inconsistencies that lead to security vulnerabilities. See Chen et al. (2002) for a discussion.
1.4.6. System Calls for Time Management
MINIX 3 has four system calls that involve the time-of-day clock. Time just returns the current time in seconds, with 0 corresponding to Jan. 1, 1970 at midnight (just as the day was starting, not ending). Of course, the system clock must be set at some point in order to allow it to be read later, so stime has been provided to let the clock be set (by the superuser). The third time call is utime, which allows the owner of a file (or the superuser) to change the time stored in a file's i-node. Application of this system call is fairly limited, but a few programs need it, for example, touch, which sets the file's time to the current time.
Finally, we have times, which returns the accounting information to a process, so it can see how much CPU time it has used directly, and how much CPU time the system itself has expended on its behalf (handling its system calls). The total user and system times used by all of its children combined are also returned.

An Overview of the Unix Filesystem

The Unix operating system design is centered on its filesystem, which has several interesting characteristics. We'll review the most significant ones, since they will be mentioned quite often.
A major function of the operating system is to hide the peculiarities of the disks and other I/O devices and present the programmer with a nice, clean abstract model of device-independent files. System calls are obviously needed to
  • create files
  • remove files
  • read files and
  • write files.
Before a file can be read, it must be opened, and after it has been read it should be closed, so calls are provided to do these things.
Figure 1-6. A file system for a university department.
To provide a place to keep files, Linux-MINIX 3 has the concept of a directory as a way of grouping files together. A student, for example, might have one directory for each course he is taking (for the programs needed for that course), another

directory for his electronic mail, and still another directory for his World Wide Web home page. System calls are then needed to create and remove directories. Calls are also provided to put an existing file into a directory, and to remove a file from a directory. Directory entries may be either files or other directories. This model also gives rise to a hierarchy the file system as shown in Fig. 1-6.
The process and file hierarchies both are organized as trees, but the similarity stops there. Process hierarchies usually are not very deep (more than three levels is unusual), whereas file hierarchies are commonly four, five, or even more levels deep. Process hierarchies are typically short-lived, generally a few minutes at most, whereas the directory hierarchy may exist for years. Ownership and protection also differ for processes and files. Typically, only a parent process may control or even access a child process, but mechanisms nearly always exist to allow files and directories to be read by a wider group than just the owner.
Every file within the directory hierarchy can be specified by giving its path name from the top of the directory hierarchy, the root directory. Such absolute path names consist of the list of directories that must be traversed from the root directory to get to the file, with slashes separating the components. In Fig. 1-6, the path for file CS101 is /Faculty/Prof.Brown/Courses/CS101. The leading slash indicates that the path is absolute, that is, starting at the root directory.
As an aside, in Windows, the backslash (\) character is used as the separator instead of the slash (/) character, so the file path given above would be written as \Faculty\Prof.Brown\Courses\CS101.
At every instant, each process has associated a current working directory, in which path names not beginning with a slash are looked for. As an example, in Fig. 1-6, if /Faculty/Prof.Brown were the working directory, then use of the path name Courses/CS101 would yield the same file as the absolute path name given above. Processes can change their working directory by issuing a system call specifying the new working directory.
Files and directories in Linux-MINIX 3 are protected by assigning each one an 11-bit binary protection code. The protection code consists of three 3-bit fields: one for the owner, one for other members of the owner's group (users are divided into groups by the system administrator), one for everyone else, and 2 bits we will discuss later. Each field has a bit for read access, a bit for write access, and a bit for execute access. These 3 bits are known as the rwx bits. For example, the protection code rwxr-x--x means that the owner can read, write, or execute the file, other group members can read or execute (but not write) the file, and everyone else can execute (but not read or write) the file. For a directory (as opposed to a file), x indicates search permission. A dash means that the corresponding permission is absent (the bit is zero).
Before a file can be read or written, it must be opened, at which time the permissions are checked. If access is permitted, the system returns a small integer called a file descriptor to use in subsequent operations. If the access is prohibited, an error code (1) is returned.
Another important concept in Linux-MINIX 3 is the mounted file system. Nearly all personal computers have one or more CD-ROM drives into which CD-ROMs can be inserted and removed. To provide a clean way to deal with removable media (CD-ROMs, DVDs, floppies, Zip drives, etc.), Linux-MINIX 3 allows the file system on a CD-ROM to be attached to the main tree. Consider the situation of Fig. 1-7(a). Before the mount call, the root file system, on the hard disk, and a second file system, on a CD-ROM, are separate and unrelated.
However, the file system on the CD-ROM cannot be used, because there is no way to specify path names on it. Linux-MINIX 3 does not allow path names to be prefixed by a drive name or number; that is precisely the kind of device dependence that operating systems ought to eliminate. Instead, the mount system call allows the file system on the CD-ROM to be attached to the root file system wherever the program wants it to be. In Fig. 1-7(b) the file system on drive 0 has been mounted on directory b, thus allowing access to files /b/x and /b/y. If directory b had originally contained any files they would not be accessible while the CD-ROM was mounted, since /b would refer to the root directory of drive 0. (Not being able to access these files is not as serious as it at first seems: file systems are nearly always mounted on empty directories.) If a system contains multiple hard disks, they can all be mounted into a single tree as well.
Another important concept in Linux-MINIX 3 is the special file. Special files are provided in order to make I/O devices look like files. That way, they can be read and written using the same system calls as are used for reading and writing files. Two kinds of special files exist: block special files and character special files. Block special files are normally used to model devices that consist of a collection of randomly addressable blocks, such as disks. By opening a block special file and reading, say, block 4, a program can directly access the fourth block on the device, without regard to the structure of the file system contained on it. Similarly, character special files are used to model printers, modems, and other devices that accept or output a character stream. By convention, the special files are kept in the /dev directory. For example, /dev/lp might be the line printer.
The last feature we will discuss in this overview is one that relates to both processes and files: pipes. A pipe is a sort of pseudofile that can be used to connect two processes, as shown in Fig. 1-8. If processes A and B wish to talk using a pipe, they must set it up in advance. When process A wants to send data to process B, it writes on the pipe as though it were an output file. Process B can read the data by reading from the pipe as though it were an input file. Thus, communication between processes in Linux-MINIX 3 looks very much like ordinary file reads and writes. Stronger yet, the only way a process can discover that the output file it is writing on is not really a file, but a pipe, is by making a special system call.

A Unix file is an information container structured as a sequence of bytes; the kernel does not interpret the contents of a file. Many programming libraries implement higher-level abstractions, such as records structured into fields and record addressing based on keys(like DB files). However, the programs in these libraries must rely on system calls offered by the kernel. From the user's point of view, files are organized in a tree-structured namespace, as shown in Figure 1-1.

All the nodes of the tree, except the leaves, denote directory names. A directory node contains information about the files and directories just beneath it. A file or directory name consists of a sequence of arbitrary ASCII characters (with the exception of / and of the null character \0). Most filesystems place a limit on the length of a filename, typically no more than 255 characters. Some operating systems allow filenames to be expressed in many different alphabets, based on 16-bit extended coding of graphical characters such as Unicode. The directory corresponding to the root of the tree is called the root directory. By convention, its name is a slash (/). Names must be different within the same directory, but the same name may be used in different directories.

Unix associates a current working directory with each process; it belongs to the process execution context, and it identifies the directory currently used by the process. To identify a specific file, the process uses a pathname, which consists of slashes alternating with a sequence of directory names that lead to the file. If the first item in the pathname is a slash, the pathname is said to be absolute, because its starting point is the root directory. Otherwise, if the first item is a directory name or filename, the pathname is said to be relative, because its starting point is the process's current directory.

While specifying filenames, the notations "." and ".." are also used. They denote the current working directory and its parent directory, respectively. If the current working directory is the root directory, "." and ".." coincide.

Hard and Soft Links
A filename included in a directory is called a file hard link (or more simply, a link). The same file may have several links included in the same directory or in different ones, so it may have several filenames.

The Unix command:

$ ln p1 p2

is used to create a new hard link that has the pathname p2 for a file identified by the pathname p1.

Hard links have two limitations:

  1. It is not possible to create hard links for directories. Doing so might transform the directory tree into a graph with cycles, thus making it impossible to locate a file according to its name.
  2. Links can be created only among files included in the same filesystem. This is a serious limitation, because modern Unix systems may include several filesystems located on different disks and/or partitions, and users may be unaware of the physical divisions between them.
To overcome these limitations, soft links (also called symbolic links) were introduced a long time ago. Symbolic links are short files that contain an arbitrary pathname of another file. The pathname may refer to any file or directory located in any filesystem; it may even refer to a non existent file.

The Unix command:

$ ln -s p1 p2

creates a new soft link with pathname p2 that refers to pathname p1. When this command is executed, the filesystem extracts the directory part of p2 and creates a new entry in that directory of type symbolic link, with the name indicated by p2. This new file contains the name indicated by pathname p1. This way, each reference to p2 can be translated automatically into a reference to p1.

File Types
Unix files may have one of the following types:
  • Regular file
  • Directory
  • Symbolic link
  • Block-oriented device file
  • Character-oriented device file
  • Pipe and named pipe (also called FIFO)
  • Socket
The first three file types are constituents of any Unix filesystem.Device files are related both to I/O devices, and to device drivers integrated into the kernel. For example, when a program accesses a device file, it acts directly on the I/O device associated with that file. Pipes and sockets are special files used for interprocess communication.

File Descriptor and Inode
Unix makes a clear distinction between the contents of a file and the information about a file. With the exception of device files and files of special filesystems, each file consists of a sequence of bytes. The file does not include any control information, such as its length or an end-of-file (EOF) delimiter.

All information needed by the filesystem to handle a file is included in a data structure called an inode. Each file has its own inode, which the filesystem uses to identify the file. While filesystems and the kernel functions handling them can vary widely from one Unix system to another, they must always provide at least the following attributes, which are specified in the POSIX standard:
  • File type (Regular, simlink etc. see the previous section)
  • Number of hard links associated with the file
  • File length in bytes
  • Device ID (i.e., an identifier of the device containing the file)
  • Inode number that identifies the file within the filesystem
  • UID of the file owner
  • User group ID of the file
  • Several timestamps that specify the inode status change time, the last access time, and the last modify time
  • Access rights and file mode (see the next section)

Access Rights and File Mode
The potential users of a file fall into three classes:
  • The user who is the owner of the file
  • The users who belong to the same group as the file, not including the owner
  • All remaining users (others)
There are three types of access rights -- read(r), write(w), and execute(x) for each of these three classes. Thus, the set of access rights associated with a file consists of nine different binary flags(3([rwx] for owner, 3 for group, 3 for others). Three additional flags, called suid (Set User ID), sgid (Set Group ID), and sticky, define the file mode. These flags have the following meanings when applied to executable files:
  • suid A process executing a file normally keeps the User ID (UID ) of the process owner. However, if the executable file has the suid flag set, the process gets the UID of the file(notice not the process) owner.
  • sgid A process executing a file keeps the user group ID of the process group. However, if the executable file has the sgid flag set, the process gets the user group ID of the file.
  • sticky An executable file with the sticky flag set corresponds to a request to the kernel to keep the program resident in memory after its execution terminates(This flag has become obsolete; other approaches based on sharing of code pages are now used).
When a file is created by a process:
  • its owner ID is the UID of the process.
  • Its owner user group ID can be either the process group ID of the creator process or the user group ID of the parent directory, depending on the value of the sgid flag of the parent directory.

File-Handling System Calls
When a user accesses the contents of either a regular file or a directory, he actually accesses some data stored in a hardware block device. In this sense, a filesystem is a user-level view of the physical organization of a hard disk partition. Because a process in User Mode cannot directly interact with the low-level hardware components, each actual file operation must be performed in Kernel Mode. Therefore, the Unix operating system defines several system calls related to file handling.

All Unix kernels devote great attention to the efficient handling of hardware block devices to achieve good overall system performance. We will describe topics related to file handling in Linux and specifically how the kernel reacts to file-related system calls. To understand those descriptions, you will need to know how the main file-handling system calls are used; these are described in the next section.

Opening a file
Processes can access only "opened" files. To open a file, the process invokes the system call:

fd = open(path, flag, mode)

The three parameters have the following meanings:
  • path Denotes the pathname (relative or absolute) of the file to be opened.
  • flag Specifies how the file must be opened (e.g., read, write, read/write, append that's kernel functions). It also can specify whether a non existing file should be created.
  • mode Specifies the access rights of a newly created file.
This system call creates an "open file" object and returns an identifier called a file descriptor. An open file object contains:
  • Some file-handling data structures, such as a set of flags specifying how the file has been opened, an offset field that denotes the current position in the file from which the next operation will take place (the so-called file pointer), and so on.
  • Some pointers to kernel functions that the process can invoke. The set of permitted functions depends on the value of the flag parameter.
We discuss open file objects in detail later. Let's limit ourselves here to describing some general properties (specified by the POSIX semantics):
  • A file descriptor represents an interaction between a process and an opened file, while an open file object contains data related to that interaction. The same open file object may be identified by several file descriptors in the same process.
  • Several processes may concurrently open the same file. In this case, the filesystem assigns a separate file descriptor to each file, along with a separate open file object. When this occurs, the Unix filesystem does not provide any kind of synchronization among the I/O operations issued by the processes on the same file. However, several system calls such as flock( ) are available to allow processes to synchronize themselves on the entire file or on portions of it.
To create a new file, the process also may invoke the creat( ) system call, which is handled by the kernel exactly like open( ).

Accessing an opened file
Regular Unix files can be addressed either sequentially or randomly, while device files and named pipes are usually accessed sequentially. In both kinds of access, the kernel stores the file pointer in the open file object that is, the current position at which the next read or write operation will take place.

Sequential access is implicitly assumed: the read( ) and write( ) system calls always refer to the position of the current file pointer. To modify the value, a program must explicitly invoke the lseek( ) system call. When a file is opened, the kernel sets the file pointer to the position of the first byte in the file (offset 0).

The lseek( ) system call requires the following parameters:

newoffset = lseek(fd, offset, whence);

which have the following meanings:
  • fd Indicates the file descriptor of the opened file
  • offset Specifies a signed integer value that will be used for computing the new position of the file pointer
  • whence Specifies whether the new position should be computed by adding the offset value to the number 0 (offset from the beginning of the file), the current file pointer, or the position of the last byte (offset from the end of the file)
The read( ) system call requires the following parameters:

nread = read(fd, buf, count);

which have the following meanings:
  • fd Indicates the file descriptor of the opened file
  • buf Specifies the address of the buffer in the process's address space to which the data will be transferred
  • count Denotes the number of bytes to read
When handling such a system call, the kernel attempts to read count bytes from the file having the file descriptor fd, starting from the current value of the opened file's offset field. In some case send-of-file, empty pipe, and so on the kernel does not succeed in reading all count bytes. The returned nread value specifies the number of bytes effectively read. The file pointer also is updated by adding nread to its previous value. The write( ) parameters are similar.

Closing a file
When a process does not need to access the contents of a file anymore, it can invoke the system call:

res = close(fd);

which releases the open file object corresponding to the file descriptor fd. When a process terminates, the kernel closes all its remaining opened files.

Renaming and deleting a file
To rename or delete a file, a process does not need to open it. Indeed, such operations do not act on the contents of the affected file, but rather on the contents of one or more directories. For example, the system call:

res = rename(oldpath, newpath);

changes the name of a file link, while the system call:

res = unlink(pathname);

decreases the file link count(count--) and removes the corresponding directory entry. The file is deleted only when the link count assumes the value 0.

An Overview of Unix Kernels
Unix kernels provide an execution environment in which applications may run. Therefore, the kernel must implement a set of services and corresponding interfaces. Applications use those interfaces and do not usually interact directly with hardware resources.

The Process/Kernel Model
As already mentioned, a CPU can run in either User Mode or Kernel Mode . Actually, some CPUs can have more than two execution states. For instance, the 80 x 86 microprocessors have four different execution states. But all standard Unix kernels use only two of them: Kernel Mode and User Mode.

When a program is executed in User Mode, it cannot directly access the kernel data structures or the kernel programs. When an application executes in Kernel Mode, however, these restrictions no longer apply. Each CPU model provides special instructions to switch from User Mode to Kernel Mode and vice versa. A program usually executes in User Mode and switches to Kernel Mode only when requesting a service provided by the kernel. When the kernel has satisfied the program's request, it puts the program back in User Mode.

Processes are dynamic entities that usually have a limited life span within the system. The task of creating, eliminating, and synchronizing the existing processes is delegated to a group of routines in the kernel.

The kernel itself is not a process but a process manager. The process/kernel model assumes that processes that require a kernel service use specific programming constructs called system calls . Each system call sets up the group of parameters that identifies the process request and then executes the hardware-dependent CPU instruction to switch from User Mode to Kernel Mode.

Besides user processes, Unix systems include a few privileged processes called kernel threads with the following characteristics:
  • They run in Kernel Mode in the kernel address space.
  • They do not interact with users, and thus do not require terminal devices.
  • They are usually created during system startup and remain alive until the system is shut down.
On a uniprocessor system, only one process is running at a time, and it may run either in User or in Kernel Mode. If it runs in Kernel Mode, the processor is executing some kernel routine. Figure 1-2 illustrates examples of transitions between User and Kernel Mode.
  1. Process 1 in User Mode issues a system call,
  2. after which the process switches to Kernel Mode, and the system call is serviced. Process 1 then resumes execution in User Mode until a timer interrupt occurs,
  3. and the scheduler is activated in Kernel Mode. A process switch takes place,
  4. and Process 2 starts its execution in User Mode until a hardware device raises an interrupt.
  5. As a consequence of the interrupt, Process 2 switches to Kernel Mode and services the interrupt.
Unix kernels do much more than handle system calls; in fact, kernel routines can be activated in several ways:
  • A process invokes a system call.
  • The CPU executing the process signals an exception, which is an unusual condition such as an invalid instruction. The kernel handles the exception on behalf of the process that caused it.
  • A peripheral device issues an interrupt signal to the CPU to notify it of an event such as a request for attention, a status change, or the completion of an I/O operation. Each interrupt signal is dealt by a kernel program called an interrupt handler. Because peripheral devices operate asynchronously with respect to the CPU, interrupts occur at unpredictable times.
  • A kernel thread is executed. Because it runs in Kernel Mode, the corresponding program must be considered part of the kernel.

Process Implementation
To let the kernel manage processes, each process is represented by a process descriptor that includes information about the current state of the process(process context). When the kernel stops the execution of a process, it saves the current contents of several processor registers in the process descriptor.

These include:
When the kernel decides to resume executing a process, it uses the proper process descriptor fields to load the CPU registers. Because the stored value of the program counter points to the instruction following the last instruction executed, the process resumes execution at the point where it was stopped.

When a process is not executing on the CPU, it is waiting for some event. Unix kernels distinguish many wait states, which are usually implemented by queues of process descriptors ; each (possibly empty) queue corresponds to the set of processes waiting for a specific event.

Reentrant Kernels
All Unix kernels are reentrant. This means that several processes may be executing in Kernel Mode at the same time. Of course, on uniprocessor systems, only one process can progress, but many can be blocked in Kernel Mode when waiting for the CPU or the completion of some I/O operation. For instance,
  1. after issuing a read to a disk on behalf of a process,
  2. the kernel lets the disk controller handle it and resumes executing other processes.
  3. An interrupt notifies the kernel when the device has satisfied the read,
  4. so the former process can resume the execution.
One way to provide reentrancy is to write functions so that they modify only local variables and do not alter global data structures. Such functions are called reentrant functions . But a reentrant kernel is not limited only to such reentrant functions (although that is how some real-time kernels are implemented). Instead, the kernel can include non-reentrant functions and use locking mechanisms to ensure that only one process can execute a non-reentrant function at a time.

If a hardware interrupt occurs, a reentrant kernel is able to suspend the current running process even if that process is in Kernel Mode. This capability is very important, because it improves the throughput of the device controllers that issue interrupts.
  1. Once a device has issued an interrupt,
  2. it waits until the CPU acknowledges it.
  3. If the kernel is able to answer quickly, the device controller will be able to perform other tasks while the CPU handles the interrupt.
Now let's look at kernel reentrancy and its impact on the organization of the kernel. A kernel control path denotes the sequence of instructions executed by the kernel to handle a system call, an exception, or an interrupt.

In the simplest case, the CPU executes a kernel control path sequentially from the first instruction to the last. When one of the following events occurs, however, the CPU interleaves the kernel control paths :
  • A process executing in User Mode invokes a system call, and the corresponding kernel control path verifies that the request cannot be satisfied immediately; it then invokes the scheduler to select a new process to run. As a result, a process (context) switch occurs. The first kernel control path is left unfinished, and the CPU resumes the execution of some other kernel control path. In this case, the two control paths are executed on behalf of two different processes.
  • The CPU detects an exception for example, access to a page not present in RAM while running a kernel control path. The first control path is suspended, and the CPU starts the execution of a suitable procedure. In our example, this type of procedure can allocate a new page for the process and read its contents from disk. When the procedure terminates, the first control path can be resumed. In this case, the two control paths are executed on behalf of the same process.
  • A hardware interrupt occurs while the CPU is running a kernel control path with the interrupts enabled. The first kernel control path is left unfinished, and the CPU starts processing another kernel control path to handle the interrupt. The first kernel control path resumes when the interrupt handler terminates. In this case, the two kernel control paths run in the execution context of the same process, and the total system CPU time is accounted to it. However, the interrupt handler doesn't necessarily operate on behalf of the process.
  • An interrupt occurs while the CPU is running with kernel preemption enabled, and a higher priority process is runnable. In this case, the first kernel control path is left unfinished, and the CPU resumes executing another kernel control path on behalf of the higher priority process. This occurs only if the kernel has been compiled with kernel preemption support.
Figure 1-3 illustrates a few examples of non-interleaved and interleaved kernel control paths. Three different CPU states are considered:
  • Running a process in User Mode (User)
  • Running an exception or a system call handler (Excp)
  • Running an interrupt handler (Intr)

Process Address Space
Each process runs in its private address space. A process running in User Mode refers to private stack, data, and code areas. When running in Kernel Mode, the process addresses the kernel data and code areas and uses another private stack.

Because the kernel is reentrant, several kernel control paths each related to a different process may be executed in turn. In this case, each kernel control path refers to its own private kernel stack.

While it appears to each process that it has access to a private address space, there are times when part of the address space is shared among processes. In some cases, this sharing is explicitly requested by processes; in others, it is done automatically by the kernel to reduce memory usage. If the same program, say an editor, is needed simultaneously by several users, the program is loaded into memory only once, and its instructions(only code not the stack & data ) can be shared by all of the users who need it. Its data, of course, must not be shared, because each user will have separate data. This kind of shared address space is done automatically by the kernel to save memory.

Processes also can share parts of their address space as a kind of interprocess communication, using the "shared memory" technique introduced in System V and supported by Linux.

Finally, Linux supports the mmap( ) system call, which allows part of a file or the information stored on a block device to be mapped into a part of a process address space. Memory mapping can provide an alternative to normal reads and writes for transferring data. If the same file is shared by several processes, its memory mapping is included in the address space of each of the processes that share it.

Synchronization and Critical Regions
Implementing a reentrant kernel requires the use of synchronization . If a kernel control path is suspended while acting on a kernel data structure, no other kernel control path should be allowed to act on the same data structure unless it has been reset to a consistent state. Otherwise, the interaction of the two control paths could corrupt the stored information.

For example, suppose a global variable V contains the number of available items of some system resource. The first kernel control path, A, reads the variable and determines that there is just one available item. At this point, another kernel control path, B, is activated and reads the same variable, which still contains the value 1. Thus, B decreases V and starts using the resource item. Then A resumes the execution; because it has already read the value of V, it assumes that it can decrease V and take the resource item, which B already uses. As a final result, V contains -1, and two kernel control paths use the same resource item with potentially disastrous effects.

When the outcome of a computation depends on how two or more processes are scheduled, the code is incorrect. We say that there is a race condition.

In general, safe access to a global variable is ensured by using atomic operations . In the previous example, data corruption is not possible if the two control paths read and decrease V with a single, non-interruptible operation. However, kernels contain many data structures that cannot be accessed with a single operation. For example, it usually isn't possible to remove an element from a linked list with a single operation, because the kernel needs to access at least two pointers at once. Any section of code that should be finished by each process that begins it before another process can enter it is called a critical region.
Synchronization problems have been fully described in other works; we refer the interested reader to books on the Unix operating systems (see the Bibliography)
These problems occur not only among kernel control paths but also among processes sharing common data. Several synchronization techniques have been adopted. The following section concentrates on how to synchronize kernel control paths.

Kernel preemption disabling

To provide a drastically simple solution to synchronization problems, some traditional Unix kernels are non-preemptive: when a process executes in Kernel Mode, it cannot be arbitrarily suspended and substituted with another process. Therefore, on a uniprocessor system, all kernel data structures that are not updated by interrupts or exception handlers are safe for the kernel to access.

Of course, a process in Kernel Mode can voluntarily relinquish the CPU, but in this case, it must ensure that all data structures are left in a consistent state. Moreover, when it resumes its execution, it must recheck the value of any previously accessed data structures that could be changed.

A synchronization mechanism applicable to preemptive kernels consists of disabling kernel preemption before entering a critical region and reenabling it right after leaving the region.
Non-preemptability is not enough for multiprocessor systems, because two kernel control paths running on different CPUs can concurrently access the same data structure.

Interrupt disabling

Another synchronization mechanism for uniprocessor systems consists of disabling all hardware interrupts before entering a critical region and reenabling them right after leaving it. This mechanism, while simple, is far from optimal. If the critical region is large, interrupts can remain disabled for a relatively long time, potentially causing all hardware activities to freeze.
Moreover, on a multiprocessor system, disabling interrupts on the local CPU is not sufficient, and other synchronization techniques must be used.

A widely used mechanism, effective in both uniprocessor and multiprocessor systems, relies on the use of semaphores . A semaphore is simply a counter associated with a data structure; it is checked by all kernel threads before they try to access the data structure. Each semaphore may be viewed as an object composed of:
  • An integer variable
  • A list of waiting processes
  • Two atomic methods: down( ) and up( )
  1. The down( ) method decreases the value of the semaphore. If the new value is less than 0, the method adds the running process to the semaphore list and then blocks (i.e., invokes the scheduler).
  2. The up( ) method increases the value of the semaphore and, if its new value is greater than or equal to 0, reactivates one or more processes in the semaphore list.
Each data structure to be protected has its own semaphore, which is initialized to 1. When a kernel control path wishes to access the data structure, it executes the down( ) method on the proper semaphore. If the value of the new semaphore isn't negative, access to the data structure is granted. Otherwise, the process that is executing the kernel control path is added to the semaphore list and blocked. When another process executes the up( ) method on that semaphore, one of the processes in the semaphore list is allowed to proceed.

Spin locks
In multiprocessor systems, semaphores are not always the best solution to the synchronization problems. Some kernel data structures should be protected from being concurrently accessed by kernel control paths that run on different CPUs. In this case, if the time required to update the data structure is short, a semaphore could be very inefficient. To check a semaphore, the kernel must insert a process in the semaphore list and then suspend it. Because both operations are relatively expensive, in the time it takes to complete them, the other kernel control path could have already released the semaphore.

In these cases, multiprocessor operating systems use spin locks . A spin lock is very similar to a semaphore, but it has no process list; when a process finds the lock closed by another process, it "spins" around repeatedly, executing a tight instruction loop until the lock becomes open.

Of course, spin locks are useless in a uniprocessor environment. When a kernel control path tries to access a locked data structure, it starts an endless loop. Therefore, the kernel control path that is updating the protected data structure would not have a chance to continue the execution and release the spin lock. The final result would be that the system hangs.

Avoiding deadlocks
Processes or kernel control paths that synchronize with other control paths may easily enter a deadlock state. The simplest case of deadlock occurs when process p1 gains access to data structure a and process p2 gains access to b, but p1 then waits for b and p2 waits for a. Other more complex cyclic waits among groups of processes also may occur. Of course, a deadlock condition causes a complete freeze of the affected processes or kernel control paths.

As far as kernel design is concerned, deadlocks become an issue when the number of kernel locks used is high. In this case, it may be quite difficult to ensure that no deadlock state will ever be reached for all possible ways to interleave kernel control paths. Several operating systems, including Linux, avoid this problem by requesting locks in a predefined order.

Signals and Interprocess Communication
Unix signals provide a mechanism for notifying processes of system events. Each event has its own signal number, which is usually referred to by a symbolic constant such as SIGTERM. There are two kinds of system events:
  • Asynchronous notifications For instance, a user can send the interrupt signal SIGINT to a foreground process by pressing the interrupt keycode (usually Ctrl-C) at the terminal.
  • Synchronous notifications For instance, the kernel sends the signal SIGSEGV to a process when it accesses a memory location at an invalid address.
The POSIX standard defines about 20 different signals, 2 of which are user-definable and may be used as a primitive mechanism for communication and synchronization among processes in User Mode. In general, a process may react to a signal delivery in two possible ways:
  1. Ignore the signal
  2. Asynchronously execute a specified procedure (the signal handler).
If the process does not specify one of these alternatives, the kernel performs a default action that depends on the signal number. The five possible default actions are:
  • Terminate the process.
  • Write the execution context and the contents of the address space in a file (core dump) and terminate the process.
  • Ignore the signal.
  • Suspend the process.
  • Resume the process's execution, if it was stopped.
Kernel signal handling is rather elaborate, because the POSIX semantics allows processes to temporarily block signals. Moreover, the SIGKILL and SIGSTOP signals cannot be directly handled by the process or ignored.

AT&T's Unix System V introduced other kinds of interprocess communication among processes in User Mode, which have been adopted by many Unix kernels:
  • semaphores ,
  • message queues , and
  • shared memory .
They are collectively known as System V IPC.

The kernel implements these constructs as IPC resources. A process acquires a resource by invoking a shmget( ) , semget( ) , or msgget( ) system call. Just like files, IPC resources are persistent: they must be explicitly deallocated by the creator process, by the current owner, or by a superuser process.

  • Semaphores are similar to those described in the section "Synchronization and Critical Regions," earlier, except that they are reserved for processes in User Mode.
  • Message queues allow processes to exchange messages by using the msgsnd( ) and msgrcv( ) system calls, which insert a message into a specific message queue and extract a message from it, respectively.
    The POSIX standard (IEEE Std 1003.1-2001) defines an IPC mechanism based on message queues, which is usually known as POSIX message queues . They are similar to the System V IPC's message queues, but they have a much simpler file-based interface to the applications.

  • Shared memory provides the fastest way for processes to exchange and share data. A process starts by issuing a shmget( ) system call to create a new shared memory having a required size. After obtaining the IPC resource identifier, the process invokes the shmat( ) system call, which returns the starting address of the new region within the process address space. When the process wishes to detach the shared memory from its address space, it invokes the shmdt( ) system call. The implementation of shared memory depends on how the kernel implements process address spaces.

Process Management
Unix makes a neat distinction between the process and the program it is executing. To that end, the fork( ) and _exit( ) system calls are used respectively to create a new process and to terminate it, while an exec( )-like system call is invoked to load a new program. After such a system call is executed, the process resumes execution with a brand new address space containing the loaded program.

The process that invokes a fork( ) is the parent, while the new process is its child. Parents and children can find one another because the data structure describing each process includes a pointer to its immediate parent and pointers to all its immediate children.

A naive implementation of the fork( ) would require both the parent's data and the parent's code to be duplicated and the copies assigned to the child. This would be quite time consuming.
Current kernels that can rely on hardware paging units follow the Copy-On-Write approach, which defers page duplication until the last moment (i.e., until the parent or the child is required to write into a page). We shall describe how Linux implements this technique in a later section "Copy On Write".

The _exit( ) system call terminates a process. The kernel handles this system call by releasing the resources owned by the process and sending the parent process a SIGCHLD signal, which is ignored by default.

Zombie processes

How can a parent process inquire about termination of its children? The wait4( ) system call allows a process to wait until one of its children terminates; it returns the process ID (PID) of the terminated child.

When executing this system call, the kernel checks whether a child has already terminated. A special zombie process state is introduced to represent terminated processes: a process remains in that state until its parent process executes a wait4( ) system call on it. The system call handler extracts data about resource usage from the process descriptor fields; the process descriptor may be released once the data is collected. If no child process has already terminated when the wait4( ) system call is executed, the kernel usually puts the process in a wait state until a child terminates.

Many kernels also implement a waitpid( ) system call, which allows a process to wait for a specific child process. Other variants of wait4( ) system calls are also quite common.

It's good practice for the kernel to keep around information on a child process until the parent issues its wait4( ) call, but suppose the parent process terminates without issuing that call? The information takes up valuable memory slots that could be used to serve living processes. For example, many shells allow the user to start a command in the background and then log out. The process that is running the command shell terminates, but its children continue their execution.
The solution lies in a special system process called init, which is created during system initialization. When a process terminates, the kernel changes the appropriate process descriptor pointers of all the existing children of the terminated process to make them become children of init. This process monitors the execution of all its children and routinely issues wait4( ) system calls, whose side effect is to get rid of all orphaned zombies.

Process groups and login sessions
Modern Unix operating systems introduce the notion of process groups to represent a "job" abstraction. For example, in order to execute the command line:

$ ls | sort | more

a shell that supports process groups, such as bash, creates a new group for the three processes corresponding to ls, sort, and more. In this way, the shell acts on the three processes as if they were a single entity (the job, to be precise). Each process descriptor includes a field containing the process group ID . Each group of processes may have a group leader, which is the process whose PID coincides with the process group ID. A newly created process is initially inserted into the process group of its parent.

Modern Unix kernels also introduce login sessions. Informally, a login session contains all processes that are descendants of the process that has started a working session on a specific terminal usually, the first command shell process created for the user. All processes in a process group must be in the same login session. A login session may have several process groups active simultaneously; one of these process groups is always in the foreground, which means that it has access to the terminal. The other active process groups are in the background. When a background process tries to access the terminal, it receives a SIGTTIN or SIGTTOUT signal. In many command shells, the internal commands bg and fg can be used to put a process group in either the background or the foreground.

Memory Management
Memory management is by far the most complex activity in a Unix kernel. More than a third of this book is dedicated just to describing how Linux handles memory management. This section illustrates some of the main issues related to memory management.

Virtual memory
All recent Unix systems provide a useful abstraction called virtual memory . Virtual memory acts as a logical layer between the application memory requests and the hardware Memory Management Unit (MMU). Virtual memory has many purposes and advantages:
  • Several processes can be executed concurrently.
  • It is possible to run applications whose memory needs are larger than the available physical memory.
  • Processes can execute a program whose code is only partially loaded in memory.
  • Each process is allowed to access a subset of the available physical memory.
  • Processes can share a single memory image of a library or program.
  • Programs can be relocatable that is, they can be placed anywhere in physical memory.
  • Programmers can write machine-independent code, because they do not need to be concerned about physical memory organization.
The main ingredient of a virtual memory subsystem is the notion of virtual address space. The set of memory references that a process can use is different from physical memory addresses. When a process uses a virtual address,[*] the kernel and the MMU cooperate to find the actual physical location of the requested memory item.
[*] These addresses have different nomenclatures, depending on the computer architecture. As we'll see, Intel manuals refer to them as "logical addresses."
Today's CPUs include hardware circuits that automatically translate the virtual addresses into physical ones. To that end, the available RAM is partitioned into page frames typically 4 or 8 KB in length and a set of Page Tables is introduced to specify how virtual addresses correspond to physical addresses. These circuits make memory allocation simpler, because a request for a block of contiguous virtual addresses can be satisfied by allocating a group of page frames having non-contiguous physical addresses.

Random access memory usage

All Unix operating systems clearly distinguish between two portions of the random access memory (RAM). A few megabytes are dedicated to storing the kernel image (i.e., the kernel code and the kernel static data structures). The remaining portion of RAM is usually handled by the virtual memory system and is used in three possible ways:
  • To satisfy kernel requests for buffers, descriptors, and other dynamic kernel data structures
  • To satisfy process requests for generic memory areas and for memory mapping of files
  • To get better performance from disks and other buffered devices by means of caches
Each request type is valuable. On the other hand, because the available RAM is limited, some balancing among request types must be done, particularly when little available memory is left. Moreover, when some critical threshold of available memory is reached and a page-frame-reclaiming algorithm is invoked to free additional memory, which are the page frames most suitable for reclaiming? As we will see, there is no simple answer to this question and very little support from theory. The only available solution lies in developing carefully tuned empirical algorithms.

One major problem that must be solved by the virtual memory system is memory fragmentation . Ideally, a memory request should fail only when the number of free page frames is too small.
However, the kernel is often forced to use physically contiguous memory areas. Hence the memory request could fail even if there is enough memory available, but it is not available as one contiguous chunk.

Kernel Memory Allocator
The Kernel Memory Allocator (KMA) is a subsystem that tries to satisfy the requests for memory areas from all parts of the system. Some of these requests come from other kernel subsystems needing memory for kernel use, and some requests come via system calls from user programs to increase their processes' address spaces. A good KMA should have the following features:
  • It must be fast. Actually, this is the most crucial attribute, because it is invoked by all kernel subsystems (including the interrupt handlers).
  • It should minimize the amount of wasted memory.
  • It should try to reduce the memory fragmentation problem.
  • It should be able to cooperate with the other memory management subsystems to borrow and release page frames from them.
Several proposed KMAs, which are based on a variety of different algorithmic techniques, include:
As we will see, Linux's KMA uses a Slab allocator on top of a buddy system.

Process virtual address space handling
The address space of a process contains all the virtual memory addresses that the process is allowed to reference. The kernel usually stores a process virtual address space as a list of memory area descriptors . For example, when a process starts the execution of some program via an exec( )-like system call, the kernel assigns to the process a virtual address space that comprises memory areas for:
  • The executable code of the program
  • The initialized data of the program
  • The uninitialized data of the program
  • The initial program stack (i.e., the User Mode stack)
  • The executable code and data of needed shared libraries
  • The heap (the memory dynamically requested by the program)
All recent Unix operating systems adopt a memory allocation strategy called demand paging . With demand paging, a process can start program execution with none of its pages in physical memory. As it accesses a non present page, the MMU generates an exception; the exception handler finds the affected memory region, allocates a free page, and initializes it with the appropriate data. In a similar fashion, when the process dynamically requires memory by using malloc( ), or the brk( ) system call (which is invoked internally by malloc( )), the kernel just updates the size of the heap memory region of the process. A page frame is assigned to the process only when it generates an exception by trying to refer its virtual memory addresses.
Virtual address spaces also allow other efficient strategies, such as the Copy On Write strategy mentioned earlier. For example, when a new process is created, the kernel just assigns the parent's page frames to the child address space, but marks them read-only. An exception is raised as soon the parent or the child tries to modify the contents of a page. The exception handler assigns a new page frame to the affected process and initializes it with the contents of the original page.

A good part of the available physical memory is used as cache for hard disks and other block devices. This is because hard drives are very slow: a disk access requires several milliseconds, which is a very long time compared with the RAM access time. Therefore, disks are often the bottleneck in system performance. As a general rule, one of the policies already implemented in the earliest Unix system is to defer writing to disk as long as possible. As a result, data read previously from disk and no longer used by any process continue to stay in RAM.

This strategy is based on the fact that there is a good chance that new processes will require data read from or written to disk by processes that no longer exist. When a process asks to access a disk, the kernel checks first whether the required data are in the cache. Each time this happens (a cache hit), the kernel is able to service the process request without accessing the disk.
The sync( ) system call forces disk synchronization by writing all of the "dirty" buffers (i.e., all the buffers whose contents differ from that of the corresponding disk blocks) into disk. To avoid data loss, all operating systems take care to periodically write dirty buffers back to disk.

Device Drivers
The kernel interacts with I/O devices by means of device drivers . Device drivers are included in the kernel and consist of data structures and functions that control one or more devices, such as hard disks, keyboards, mouses, monitors, network interfaces, and devices connected to an SCSI bus. Each driver interacts with the remaining part of the kernel (even with other drivers) through a specific interface. This approach has the following advantages:
  • Device-specific code can be encapsulated in a specific module.
  • Vendors can add new devices without knowing the kernel source code; only the interface specifications must be known.
  • The kernel deals with all devices in a uniform way and accesses them through the same interface.
  • It is possible to write a device driver as a module that can be dynamically loaded in the kernel without requiring the system to be rebooted.
  • It is also possible to dynamically unload a module that is no longer needed, therefore minimizing the size of the kernel image stored in RAM.
Figure 1-4 illustrates how device drivers interface with the rest of the kernel and with the processes.

Some user programs (P) wish to operate on hardware devices. They make requests to the kernel using the usual file-related system calls and the device files normally found in the /dev directory. Actually, the device files are the user-visible portion of the device driver interface. Each device file refers to a specific device driver, which is invoked by the kernel to perform the requested operation on the hardware component.

At the time Unix was introduced, graphical terminals were uncommon and expensive, so only alphanumeric terminals were handled directly by Unix kernels. When graphical terminals became widespread, ad hoc applications such as the X Window System were introduced that ran as standard processes and accessed the I/O ports of the graphics interface and the RAM video area directly. Some recent Unix kernels, such as Linux 2.6, provide an abstraction for the frame buffer of the graphic card and allow application software to access them without needing to know anything about the I/O ports of the graphics interface (see the section "Levels of Kernel Support" later)

Memory Addressing

This chapter deals with addressing techniques. Luckily, an operating system is not forced to keep track of physical memory all by itself; today's microprocessors include several hardware circuits to make memory management both more efficient and more robust so that programming errors cannot cause improper accesses to memory outside the program.

Here we offer details in this chapter on how 80 x 86 microprocessors address memory chips and how Linux uses the available addressing circuits. You will find, we hope, that when you learn the implementation details on Linux's most popular platform you will better understand both the general theory of paging and how to research the implementation on other platforms.

Memory Addresses
Programmers casually refer to a memory address as the way to access the contents of a memory cell. But when dealing with 80 x 86 microprocessors, we have to distinguish three kinds of addresses:

  • Logical address Included in the machine language instructions to specify the address of an operand or of an instruction. This type of address embodies the well-known 80 x 86 segmented architecture that forces MS-DOS and Windows programmers to divide their programs into segments . Each logical address consists of a segment and an offset (or displacement) that denotes the distance from the start of the segment to the actual address.
  • Linear address (also known as virtual address) A single 32-bit unsigned integer that can be used to address up to 4 GB that is, up to 4,294,967,296 memory cells. Linear addresses are usually represented in hexadecimal notation; their values range from 0x00000000 to 0xffffffff.
  • Physical address Used to address memory cells in memory chips. They correspond to the electrical signals sent along the address pins of the microprocessor to the memory bus. Physical addresses are represented as 32-bit or 36-bit unsigned integers.
The Memory Management Unit (MMU) transforms a logical address into a linear address by means of a hardware circuit called a segmentation unit ; subsequently, a second hardware circuit called a paging unit transforms the linear address into a physical address (see Figure 2-1).

In multiprocessor systems, all CPUs usually share the same memory; this means that RAM chips may be accessed concurrently by independent CPUs. Because read or write operations on a RAM chip must be performed serially, a hardware circuit called a memory arbiter is inserted between the bus and every RAM chip. Its role is to grant access to a CPU if the chip is free and to delay it if the chip is busy servicing a request by another processor. Even uniprocessor systems use memory arbiters , because they include specialized processors called DMA controllers that operate concurrently with the CPU (see the section "Direct Memory Access (DMA)"). In the case of multiprocessor systems, the structure of the arbiter is more complex because it has more input ports. The dual Pentium, for instance, maintains a two-port arbiter at each chip entrance and requires that the two CPUs exchange synchronization messages before attempting to use the common bus. From the programming point of view, the arbiter is hidden because it is managed by hardware circuits.

Segmentation in Hardware
Starting with the 80286 model(was my first one), Intel microprocessors perform address translation in two different ways called real mode and protected mode . We'll focus in the next sections on address translation when protected mode is enabled. Real mode exists mostly to maintain processor compatibility with older models and to allow the operating system to bootstrap.

Segment Selectors and Segmentation Registers
A logical address consists of two parts:
  1. A segment identifier: The segment identifier is a 16-bit field called the Segment Selector (see Figure 2-2)
  2. An offset: specifies the relative address within the segment. Is a 32-bit field
We'll describe the fields of Segment Selectors in the section "Fast Access to Segment Descriptors" later in this section.

To make it easy to retrieve segment selectors quickly, the processor provides segmentation registers whose only purpose is to hold Segment Selectors; these registers are called cs, ss, ds, es, fs, and gs. Although there are only six of them, a program can reuse the same segmentation register for different purposes by saving its content in memory and then restoring it later.

Three of the six segmentation registers have specific purposes:
  • cs The code segment register, which points to a segment containing program instructions
  • ss The stack segment register, which points to a segment containing the current program stack
  • ds The data segment register, which points to a segment containing global and static data
The remaining three segmentation registers are general purpose and may refer to arbitrary data segments.

The cs register has another important function: it includes a 2-bit field(2^2=4 values) that specifies the Current Privilege Level (CPL) of the CPU. The value 0 denotes the highest privilege level, while the value 3 denotes the lowest one. Linux uses only levels 0 and 3, which are respectively called Kernel Mode and User Mode.

Segment Descriptors
Each segment is represented by an 8-byte Segment Descriptor that describes the segment characteristics. Segment Descriptors are stored either in the Global Descriptor Table (GDT ) or in the Local Descriptor Table(LDT).

Usually only one GDT is defined, while each process is permitted to have its own LDT if it needs to create additional segments besides those stored in the GDT. The address and size of the GDT in main memory are contained in the gdtr control register, while the address and size of the currently used LDT are contained in the ldtr control register.

Figure 2-3 illustrates the format of a Segment Descriptor; the meaning of the various fields is explained in Table 2-1.

Table 2-1. Segment Descriptor fields
Field name Description
  • Base Contains the linear address of the first byte of the segment.
  • G Granularity flag: if it is cleared (equal to 0), the segment size is expressed in bytes; otherwise, it is expressed in multiples of 4096 bytes.
  • Limit Holds the offset of the last memory cell in the segment, thus binding the segment length. When G is set to 0, the size of a segment may vary between 1 byte and 1 MB; otherwise, it may vary between 4 KB and 4 GB.
  • S System flag: if it is cleared, the segment is a system segment that stores critical data structures such as the Local Descriptor Table; otherwise, it is a normal code or data segment.
  • Type Characterizes the segment type and its access rights (see the text that follows this table).
  • DPL Descriptor Privilege Level: used to restrict accesses to the segment. It represents the minimal CPU privilege level requested for accessing the segment. Therefore, a segment with its DPL set to 0 is accessible only when the CPL is 0 that is, in Kernel Mode while a segment with its DPL set to 3 is accessible with every CPL value.
  • P Segment-Present flag : is equal to 0 if the segment is not stored currently in main memory. Linux always sets this flag (bit 47) to 1, because it never swaps out whole segments to disk.
  • D or B Called D or B depending on whether the segment contains code or data. Its meaning is slightly different in the two cases, but it is basically set (equal to 1) if the addresses used as segment offsets are 32 bits long, and it is cleared if they are 16 bits long (see the Intel manual for further details).
  • AVL May be used by the operating system, but it is ignored by Linux.
There are several types of segments, and thus several types of Segment Descriptors. The following list shows the types that are widely used in Linux.
  • Code Segment Descriptor Indicates that the Segment Descriptor refers to a code segment; it may be included either in the GDT or in the LDT. The descriptor has the S flag set (non-system segment).
  • Data Segment Descriptor Indicates that the Segment Descriptor refers to a data segment; it may be included either in the GDT or in the LDT. The descriptor has the S flag set. Stack segments are implemented by means of generic data segments.
Task State Segment Descriptor (TSSD) Indicates that the Segment Descriptor refers to a Task State Segment (TSS) that is, a segment used to save the contents of the processor registers (see the section "Task State Segment"); it can appear only in the GDT. The corresponding Type field has the value 11 or 9, depending on whether the corresponding process is currently executing on a CPU. The S flag of such descriptors is set to 0.

Local Descriptor Table Descriptor (LDTD) Indicates that the Segment Descriptor refers to a segment containing an LDT; it can appear only in the GDT. The corresponding Type field has the value 2. The S flag of such descriptors is set to 0. The next section shows how 80 x 86 processors are able to decide whether a segment descriptor is stored in the GDT or in the LDT of the process.

Fast Access to Segment Descriptors

We recall that logical addresses consist of a 16-bit Segment Selector and a 32-bit Offset, and that segmentation registers store only the Segment Selector.

To speed up the translation of logical addresses into linear addresses, the 80 x 86 processor provides an additional non programmable register that is, a register that cannot be set by a programmer for each of the six programmable segmentation registers. Each non programmable register contains the 8-byte Segment Descriptor (described in the previous section) specified by the Segment Selector contained in the corresponding segmentation register. Every time a Segment Selector is loaded in a segmentation register, the corresponding Segment Descriptor is loaded from memory into the matching non programmable CPU register. From then on, translations of logical addresses referring to that segment can be performed without accessing the GDT or LDT stored in main memory; the processor can refer only directly to the CPU register containing the Segment Descriptor. Accesses to the GDT or LDT are necessary only when the contents of the segmentation registers change (see Figure 2-4). Any Segment Selector includes three fields that are described in Table 2-2.

Table 2-2. Segment Selector fields

Field name Description
  • index Identifies the Segment Descriptor entry contained in the GDT or in the LDT (described further in the text following this table).
  • TI Table Indicator : specifies whether the Segment Descriptor is included in the GDT (TI = 0) or in the LDT (TI = 1).
  • RPL Requestor Privilege Level : specifies the Current Privilege Level of the CPU when the corresponding Segment Selector is loaded into the cs register; it also may be used to selectively weaken the processor privilege level when accessing data segments (see Intel documentation for details).
Because a Segment Descriptor is 8 bytes long, its relative address inside the GDT or the LDT is obtained by multiplying the 13-bit index field of the Segment Selector by 8. For instance, if the GDT is at 0x00020000 (the value stored in the gdtr register) and the index specified by the Segment Selector is 2, the address of the corresponding Segment Descriptor is 0x00020000 + (2 x 8), or 0x00020010.

The first entry of the GDT is always set to 0. This ensures that logical addresses with a null Segment Selector will be considered invalid, thus causing a processor exception. The maximum number of Segment Descriptors that can be stored in the GDT is 8,191 (i.e., 213-1).

Segmentation Unit
Figure 2-5 shows in detail how a logical address is translated into a corresponding linear address. The segmentation unit performs the following operations:

  • Examines the TI field of the Segment Selector to determine which Descriptor Table stores the Segment Descriptor. This field indicates that the Descriptor is either in the GDT (in which case the segmentation unit gets the base linear address of the GDT from the gdtr register) or in the active LDT (in which case the segmentation unit gets the base linear address of that LDT from the ldtr register).
  • Computes the address of the Segment Descriptor from the index field of the Segment Selector. The index field is multiplied by 8 (the size of a Segment Descriptor), and the result is added to the content of the gdtr or ldtr register.
  • Adds the offset of the logical address to the Base field of the Segment Descriptor, thus obtaining the linear address.
Notice that, thanks to the non programmable registers associated with the segmentation registers, the first two operations need to be performed only when a segmentation register has been changed.

A deeper view

Introduction to Processes
All modern computers can do several things at the same time. In a multiprogramming system, the CPU also switches from program to program, running each for tens or hundreds of milliseconds. While, strictly speaking, at any instant of time, the CPU is running only one program, in the course of 1 second, it may work on several programs, thus giving the users the illusion of parallelism. Sometimes people speak of pseudo-parallelism in this context, to contrast it with the true hardware parallelism of multiprocessor systems (which have two or more CPUs sharing the same physical memory). Keeping track of multiple, parallel activities is hard for people to do. Therefore, operating system designers over the years have evolved a conceptual model (sequential processes) that makes parallelism easier to deal with.

The Process Model
In this model, all the runnable software on the computer, sometimes including the operating system, is organized into a number of sequential processes, or just processes for short. A process is just an executing program, including the current values of the program counter, registers, and variables(the so called context). Conceptually, each process has its own virtual CPU. In reality, of course, the real CPU switches back and forth from process to process, but to understand the system, it is much easier to think about a collection of processes running in (pseudo) parallel, than to try to keep track of how the CPU switches from program to program. This rapid switching back and forth is called multiprogramming.
In Fig. 2-1(a) we see a computer multiprogramming four programs in memory. In Fig. 2-1(b) we see four processes, each with its own flow of control (i.e., its own program counter), and each one running independently of the other ones. Of course, there is only one physical program counter, so when each process runs, its logical program counter is loaded into the real program counter. When it is finished for the time being, the physical program counter is saved in the process' logical program counter in memory. In Fig. 2-1(c) we see that viewed over a long enough time interval, all the processes have made progress, but at any given instant only one process is actually running.
With the CPU switching back and forth among the processes, the rate at which a process performs its computation will not be uniform, and probably not even reproducible if the same processes are run again. Thus, processes must not be programmed with built-in assumptions about timing. Consider, for example, an I/O process that starts a streamer tape to restore backed up files, executes an idle loop 10,000 times to let it get up to speed, and then issues a command to read the first record. If the CPU decides to switch to another process during the idle loop, the tape process might not run again until after the first record was already past the read head. When a process has critical real-time requirements like this, that is, particular events must occur within a specified number of milliseconds, special measures must be taken to ensure that they do occur. Normally, however, most processes are not affected by the underlying multiprogramming of the CPU or the relative speeds of different processes.

The difference between a process and a program is subtle, but crucial. The key idea here is that a process is an activity of some kind. It has a program, input, output, and a state. A single processor may be shared among several processes, with some scheduling algorithm being used to determine when to stop work on one process and service a different one.

Process Creation
Operating systems need some way to make sure all the necessary processes exist. In very simple systems, or in systems designed for running only a single application (e.g., controlling a device in real time), it may be possible to have all the processes that will ever be needed be present when the system comes up. In general-purpose systems, however, some way is needed to create and terminate processes as needed during operation. We will now look at some of the issues. There are four principal events that cause processes to be created:
  1. System initialization.
  2. Execution of a process creation system call by a running process.
  3. A user request to create a new process.
  4. Initiation of a batch job.
When an operating system is booted, often several processes are created. Some of these are foreground processes, that is, processes that interact with (human) users and perform work for them. Others are background processes, which are not associated with particular users, but instead have some specific function. For example, a background process may be designed to accept incoming requests for web pages hosted on that machine, waking up when a request arrives to service the request. Processes that stay in the background to handle some activity such as web pages, printing, and so on are called daemons. Large systems commonly have dozens of them.
In MINIX 3, the ps program can be used to list the running processes.
In addition to the processes created at boot time, new processes can be created afterward as well. Often a running process will issue system calls to create one or more new processes to help it do its job. Creating new processes is particularly useful when the work to be done can easily be formulated in terms of several related, but otherwise independent interacting processes. For example, when compiling a large program, the make program invokes the C compiler to convert source files to object code, and then it invokes the install program to copy the program to its destination, set ownership and permissions, etc.
In MINIX 3, the C compiler itself is actually several different programs, which work together. These include a preprocessor, a C language parser, an assembly language code generator, an assembler, and a linker.
In interactive systems, users can start a program by typing a command. In MINIX 3, virtual consoles allow a user to start a program, say a compiler, and then switch to an alternate console and start another program, perhaps to edit documentation while the compiler is running.
The last situation in which processes are created applies only to the batch systems found on large mainframes. Here users can submit batch jobs to the system (possibly remotely). When the operating system decides that it has the resources to run another job, it creates a new process and runs the next job from the input queue in it.
Technically, in all these cases, a new process is created by having an existing process execute a process creation system call. That process may be a running user process, a system process invoked from the keyboard or mouse, or a batch manager process. What that process does is execute a system call to create the new process. This system call tells the operating system to create a new process and indicates, directly or indirectly, which program to run in it.
In MINIX 3, there is only one system call to create a new process: fork. This call creates an exact clone of the calling process. After the fork, the two processes, the parent and the child, have the same memory image, the same environment strings, and the same open files. That is all there is. Usually, the child process then executes execve or a similar system call to change its memory image and run a new program. For example, when a user types a command, say, sort, to the shell, the shell forks off a child process and the child executes sort. The reason for this two-step process is to allow the child to manipulate its file descriptors after the fork but before the execve to accomplish redirection of standard input, standard output, and standard error.

In both MINIX 3 and UNIX, after a process is created both the parent and child have their own distinct address spaces. If either process changes a word in its address space, the change is not visible to the other process. The child's initial address space is a copy of the parent's, but there are two distinct address spaces involved; no writable memory is shared (like some UNIX implementations, MINIX 3 can share the program text between the two since that cannot be modified). It is, however, possible for a newly created process to share some of its creator's other resources, such as open files.

Process Termination
After a process has been created, it starts running and does whatever its job is. However, nothing lasts forever, not even processes. Sooner or later the new process will terminate, usually due to one of the following conditions:
  1. Normal exit (voluntary): Most processes terminate because they have done their work.
  2. Error exit (voluntary): When a compiler has compiled the program given to it, the compiler executes a system call to tell the operating system that it is finished. This call is exit in MINIX 3. Screen-oriented programs also support voluntary termination. For instance, editors always have a key combination that the user can invoke to tell the process to save the working file, remove any temporary files that are open and terminate.
    The second reason for termination is that the process discovers a fatal error. For example, if a user types the command

    $ cc foo.c

    to compile the program foo.c and no such file exists, the compiler simply exits.
  3. Fatal error (involuntary): The third reason for termination is an error caused by the process, perhaps due to a program bug. Examples include executing an illegal instruction, referencing non existent memory, or dividing by zero. In MINIX 3, a process can tell the operating system that it wishes to handle certain errors itself, in which case the process is signaled (interrupted) instead of terminated when one of the errors occurs.
  4. Killed by another process (involuntary): The fourth reason a process might terminate is that one process executes a system call telling the operating system to kill some other process. In MINIX 3, this call is kill. Of course, the killer must have the necessary authorization to do in the killee. In some systems, when a process terminates, either voluntarily or otherwise, all processes it created are immediately killed as well. MINIX 3 does not work this way, however.

Process Hierarchies
In some systems, when a process creates another process, the parent and child continue to be associated in certain ways. The child can itself create more processes, forming a process hierarchy. A process(except init) has only one parent (but zero, one, two, or more children).
In MINIX 3, a process, its children, and further descendants together may form a process group. When a user sends a signal from the keyboard, the signal may be delivered to all members of the process group currently associated with the keyboard (usually all processes that were created in the current window). This is signal-dependent. If a signal is sent to a group, each process can
  • catch the signal
  • ignore the signal or
  • take the default action (which is to be killed by the signal)
As a simple example of how process trees are used, let us look at how MINIX 3 initializes itself. Two special processes, the reincarnation server and init are present in the boot image. The reincarnation server's job is to (re)start drivers and servers. It begins by blocking, waiting for a message telling it what to create.
In contrast, init executes the /etc/rc script that causes it to issue commands to the reincarnation server to start the drivers and servers not present in the boot image. This procedure makes the drivers and servers so started children of the reincarnation server, so if any of them ever terminate, the reincarnation server will be informed and can restart (i.e., reincarnate) them again. This mechanism is intended to allow MINIX 3 to tolerate a driver or server crash because a new one will be started automatically. In practice, replacing a driver is much easier than replacing a server, however, since there fewer repercussions elsewhere in the system. (And, we do not say this always works perfectly; it is still work in progress.)
When init has finished this, it reads a configuration file /etc/ttytab to see which terminals and virtual terminals exist. Init forks a getty process for each one, displays a login prompt on it, and then waits for input. When a name is typed,
  1. getty execs a login process with the name as its argument.
  2. If the user succeeds in logging in, login will exec the user's shell. So the shell is a child of init.
  3. User commands create children of the shell, which are grandchildren of init.
This sequence of events is an example of how process trees are used.

Process States
Although each process is an independent entity, with its own program counter registers, stack, open files, alarms, and other internal state, processes often need to interact, communicate, and synchronize with other processes. One process may generate some output that another process uses as input, for example. In that case, the data needs to be moved between processes. In the shell command

$ cat chapter1 chapter2 chapter3 | grep tree 

  1. the first process, running cat, concatenates and outputs three files.
  2. The second process, running grep, selects all lines containing the word "tree."
Depending on the relative speeds of the two processes (which depends on both the relative complexity of the programs and how much CPU time each one has had), it may happen that grep is ready to run, but there is no input waiting for it. It must then block until some input is available.
  1. When a process blocks, it does so because logically it cannot continue, typically because it is waiting for input that is not yet available: the suspension is inherent in the problem (you cannot process the user's command line until it has been typed)
  2. It is also possible for a process that is conceptually ready and able to run to be stopped because the operating system has decided to allocate the CPU to another process for a while.That's a technicality of the system (not enough CPUs to give each process its own private processor)
These two conditions are completely different. In Fig. 2-2 we see a state diagram showing the three states a process may be in:
1. Running (actually using the CPU at that instant).
2. Ready (runnable; temporarily stopped to let another process run).
3. Blocked (unable to run until some external event happens).


Operating Systems Design and Implementation, Third Edition
By Andrew S. Tanenbaum - Vrije Universiteit Amsterdam, The Netherlands, Albert S. Woodhull - Amherst, Massachusetts
Publisher: Prentice Hall
Pub Date: January 04, 2006
ISBN-10: 0-13-142938-8

Understanding the Linux Kernel, 3rd Edition
By Daniel P. Bovet, Marco Cesati
Publisher: O'Reilly
Pub Date: November 2005
ISBN: 0-596-00565-2

Operating Systems: Concurrent and Distributed Software Design
By Jean Bacon, Tim Harris
Publisher: Addison Wesley
Pub Date: March 11, 2003
ISBN: 0-321-11789-1

The Design and Implementation of the FreeBSD Operating System
By Marshall Kirk McKusick, George V. Neville-Neil
Publisher : Addison Wesley
Pub Date : August 02, 2004
ISBN : 0-201-70245-2

Migrating to the Solaris Operating System: The Discipline of UNIX-to-UNIX® Migrations
By Ken Pepple, Brian Down, David Levy
Publisher: Prentice Hall PTR
Pub Date: November 05, 2003
ISBN: 0-13-150263-8