THE STUDY OF OPERATING SYSTEMS

 There has never been a more interesting time to study operating systems, and it has never been easier. The open-source movement has overtaken operating system, causing many of them to be made available in both source and binary (executable) format. The list of operating systems available in both formats includes Linux. BSD UNIX, Solaris, and part of macOS. The availability of source code allows us to study operating systems from the inside out. Questions that we could once answer only by looking at documentation or the behavior of an operating system we can now answer by examining the code itself.

Operating systems that are no longer commerically viable have been open-souced as well, enabling us to study how systems operated in a time of fewer CPU, memory, and storage resources. An extensive but incomplete list of open-soiurce operating-system projects is available from http://dmoz.org/Computers/Software/Operating.Systems/Oepn.Sources/. computer function makes it possible to run many operating systems on top of one core system. For example, VMware (http://www.virtualbox.com) provides a free, open-source virtual machine manager on many operating systems. Using such tools, students can try out hundreds of operating systems without dedicated hardware.

In some cases, simulators of specific hardware are also available, allowing the operating system to run on "native" hardware, all within the confines of a modern computer and modern operating system. For example, a DECSYSTEM-20 simulator running ono macOS can boot TOPS-20, load the source topes, and modify and comile a new TOPS-20 kernel. An interested student can search the Internet to find the original papers that describe the operating system, as well as the original manuals.

The advent of open-source operating systems has also made it easier to make the move from student to operating-system distribution. Not so many years ago, it was difficult or impossible to get access to source code. Now, such access is limited only by how much interest, time, and disk space a student has.

1.11.4 BSD UNIX

 BSD UNIX has a longer and more complicated history than Linux. It started in 1978 as a derivative of AT&T's UNIX. Releases from the University of California at Berkeley (UCB) came in source and binary from, but they were not open source because alicense from AT&T was required. BSD UNIX's development was slowed by a lawsuit by AT&T, but eventually a fully functional, open-source version, 4.4BSD-lite, was released in 1994.

Just as with Linux, there are many distributions of BSD UNIX, including FreeBSD, NetBSD, OpenBSD, and DragonflyBSD. To explore the source code of ReeBSD, simply download the virtual machine image of the version of interest and boot it within Virtualbox, as described above for Linux. The source code comes with the distribution and is stored in /usr/src/. The kernel source code is in /usr/src/sys. For example, to examine the virtual memory implementation code in the FreeBSD kernel, see the files in /usr/src/sys/vm. Alternatively, you can simple view the source code online at https://svnweb.freebsd.org.

As with amnyu open-source projects, this source code is contained in and controlled by a version control system-in this case, "subversion" (https://subversion.apache.org/source-code). Version control systems allow a user to "pull" an entire source code tree to his computer and "push" any changes back into the repository for others to then pull. These systems also provide other features, including an entire history of each file and a conflict resolution feature in case the same file is changed concurrently. Another version control system is git, which is used for GNU/Linux, as well as other programs (https://www.git-scm.com).

Darwin, the core kernel component of macOS, is based on BSD UNIX and is open-sourced as well. That source code is available from http://www.opensource.apple.com/. Every macOS release has its open-source components posted at that site. The name of the package that contains the kernel begins with "xnu". Apple also provides extensive developer tools, documentation, and support at http://developer.apple.com.

1.11.3 GNU/Linux

 As an Example of a free and open-source operating system, consider GNU/Linux. By 1991, the GNU operating system was nearly complete. The GNU Project had developed compilers, editors, utilities, libraries, and games - whatever parts it could not find dlsewhere. However, the GNU kernel never became ready for prime time. In 1991, a student in Finland, Linus Torvalds, released a rudimentary UNIX-like kernel using the GNU compilers and tools and invited contributions worldwide. The advent of the Internet meant that anyone interested could download the source code, modify it, and submit changes to Torvalds. REleasing updates once a week allowed this so-called "Linux" operating system to grow rapidly, enhanced by several thousand programmers. In 1991, Linux was not free software, as its license permitted only noncommercial redistribution. In 1992, however, Torvalds rereleased Linux under th GPL, making it free software (and also, to use a term coined later, "open source")

The resulting GNU/Linux operating system (with the kernel properly called Linux but the full operating system including GNU tools called GNU/Linux) has spawned hundreds of unique distributions, or cutom builds, of the system. Major distributions include Red Hat, SUSE, Fedora, Devian, Slackware, and Ubuntu. Distributions vary in function, utility, installed applications, hardware support, user interface, and purpose. For example, Red Hat Enterprise Linux is geared to large commercial use. PCLinuxOS-called PCLinuxOS Supergamer DVD - is a live DVD that includes graphics drivers and games. A gamer can run it on any compatible system simply by booting from the DVD. When the gamer is finished, a reboot of the system resets it to its installed operating system.

You can run Linux on a Windows(or other) system using the following simple, fre approach:

1. Downlaod the free Virtualbox VMM tool from

https://www.virtualbox.org/

and install it on your system.

2. Choose to install an operating system from scratch, based on an installation image like a CD, or choose pre-built operating-system images that can be installed and run more quickly from a site liek

http://virtualboxes.org/images/

These image are preinstalled with operating system and application and include many flavors of GNU/Linux.

3. Boot the virtual machine within Virtuallbox.

An alternative to using Virtualbox is to use the free program Qemu(http://wiki.qumu.org/Download/), which includes the qemu-img command for converting Virtaulbox images to Qemu images to easily import them.

With this text, we provide a virtual machine image of GNU/Linux running the Ubuntu release. This iamge contains the GNU/Linux source code as well as tools for software developement. We cover examples involving the GNU/Linux image throughout this text, as well as in a detailed case study in Chapter 20.

1.11.2 Free Operating Systems

 To counter the move to limit software use and redistribution, Richard Stallman in 1984 started developing a free, UNIX-compatible operating system called GNU(which is a recursive acronym for "GNU's Not Unix!"). To Stallman, "free" refers to freedom of use, not price. The free-software movement does not object to trading a copy for an amount of money but holds that users are entitled to four certain freedoms: (1) to freely run the program, (2) to study and change the source code, and to give or sell copies either (3) with or (4) without changes. In 1985, Stallman published the GNU Manifesto, which argues that all software should be free. He also formed the Free Software Foundation (FSF) with the goal of encouraging the use and developement of free software.

The FSF uses the copyrights on its programs to implement "copyleft," a form of licensing invented by Stallman. Copylefting a work gives anyone that prossesses a copy of the work the four essential freedoms that make the work free, with the condition that redistribution must preserve these freedoms. The GNU General Publish License (GPL) is a common license under which free software is released. Foudamentally, the GPL requires that the source code be distributed with any binaries and that all copies (including modified versions) be released under the same GPL license. The Creative Commons "Attribution Sharealike" license is also a copyleft license; "sharealike" is another way of stating the idea of copyleft.

1.11.1 History

 In the early days of modern computing (that is, the 1950s), software generally came with source code. The original hackers (computer enthusiasts) at MIT's came Tech Model Railroad Club left their programs in drawers for others to work on. "Homebrew" user groups exchange code during their meetings. Company-specific user groups, such as Digital Equipement Corporation's DECUS, accepted contributions of source-code programs, collected  them onto tapes, and distributed the tapes to interested members. In 1970, Digital's operating systems were distributed as source code with no restrictions or copyright notice.

Computer and software companies eventually sought to limit the use of the binary files compiled from the source code, rather than the source code itself, helped them to achieve this goal, as well as protecting their code and their ideas from their competitors. Although the Homebrew user groups of the 1970s exchanged code during their meetings, the operating systems for hobbyist machines(such as CPM) were proprietary. By 1980, proprietary software was the usual case.

1.11 Free and Open-Source Operating Systems

 The study of operating systems has been made easier by the availability of a vast number of free software and open-source releases. Both free operating systems and open-source operating systems are available in source-code format rather than as compiled binary code. Note, though, that free software and open-source software are two different ideas championed by different groups of people (see http://gnu.rog/phliosophy/open-source-misses-the-point.html/ for a discussion on the topic). Free software (sometimes referred to as freellibresoftware) not only makes source code available but also is licensed to allow no-cost use, redistribution, and modification. Open-source software does not necessarily offer such licensing. Thus, although all free sotrware is open source, some open-source software is not "free." GNU/Linux is the most famous open-source operating system, with some distributions free and others open source only (http://www.gnu.org/distros/). Microsoft Windows is a well-known example of the opposite closed-cource approach. Wiundows is proprietary software-Microsoft owns it, restricts its use, and carefully protects its source code. Apple's macOS opoerating system comprises a hybrid approach. It contains an open-source kernel named Darwin but includes proprietary, closed-source components as well.

Starting with the source code allows the programmer to produce binary code that can be executed on a system. Doing the opposite-reverse engineering the source code from the binaries-is quite a lot of work, and useful items such as comments are never recovered. Learning operating systems by examining the source code has other benefits as well. With the source code in hand, a student can modify the operating system and then compile and run the code to try out these changes, which is an excellent learning tool. This text includes projects that involve modifying operating-system source code, while also describing algorithms at a high level to be sure all important operating-system topics are covered. Throughout the text, we provide pointers to examples of open-source code for deeper study.

There are many benefits to open-source operating systems, including a community of interested(and usually unpaid)programmers who contribute to the code by helping to write it, debug it, analyze it, provide support, and suggest changes. Arguably, open-source code is more secure than closed-source code because many more eyes are viewing the code. Certainly, open-source code has bugs, but open-source advocates argue that bugs tend to be found and fixed faster owing to the nbumber of people using and viewing the code. Companies that earn revenue from selling their programs ofter hesitate to open-source their code, but RED Hat and a myriad of other companies are doing just that and showing that commercial companies benefit, rather than suffer, when they open-source their code. Revenue can be generated though support contracts and the sale of hardware on which the software runs, for example.

1.10.6 Real-Time Embedded Systems

 Embedded computers are the most prevalent form of computers in existence. These devices are found everywhere, from car engines and manufacturing robots to optical drives and microwave ovens. They tend to have very specific tasks. The systems they run on are usually primitive, and so the operating, pefferring to spend their time monitoring and managing hardware devices, such as automobile engines are robotic arms.

These embedded systems vary considerably. Some are general-purpose computers, running standard operating systems-such as Linux-with special-purpose applications to implement the functionality. Others are hardware devices with a special-purpose embedded operating system providing just the functionality desired. Yet others are hardware devices with application-specific integrated circuits(ASICs) that perform their tasks without an operating system.

The use of embedded system continues to expand. The power of these devices, both as standalone units and as elements of networks and the web, is sure to increase as well. Even now, entire house can be computerized, so that a cetral computer-either a general-purpose computer or an embedded system-can access can enable a home owner to tell the house to heat up before she arrives home. Someday, the refrigerator will be able to notify the grocery store when it notices the milk is gone.

Embedded systems almost always run real-time operating systems. A realtime system is used when rigid time requirements have been placed on the operation of a processor or the flow of data; thus, it is often used as a control device ina dedicated applicaiton. Sensors bring data to the computer. The computer must analyze the data and possibly adjust controls to modify the sensor inputs. Systems that control scientfic experiments, medical imaging systems, industrial control systems, and certain display systems are real-time systems. Some automobile-engine fuel-injection systems, home-appliance controllers, and weapon systems are also real-time systems.

A real-time system has well-defined, fixed time constraints. Processing must be done within the defined constraints, or the system will fail. For instance, it would not do for a robot arm to be instructed to halt after it had smashed into the car it was building. A real-time system functions correctly only fi ti return the correct result within tis time constraints. Contrast this system with a traditional laptop system where it is desirable(but not mandatory) to respond quickly.

In Chapter 5, we consider the scheduling facility needed to implement realtime functionality in an operating system, and in Chapter 20 we describe the real-time components of Linux.