Examples of operating systems

Microsoft Windows

Microsoft Windows is a family of proprietary operating systems that originated as an add-on to the older MS-DOS operating system for the IBM PC. Modern versions are based on the newer Windows NT kernel that was originally intended for OS/2. Windows runs on x86, x86-64 and Itanium processors. Earlier versions also ran on the DEC Alpha, MIPS, Fairchild (later Intergraph) Clipper and PowerPC architectures (some work was done to port it to the SPARC architecture).

As of June 2008, Microsoft Windows holds a large amount of the worldwide desktop market share. Windows is also used on servers, supporting applications such as web servers and database servers. In recent years, Microsoft has spent significant marketing and research & development money to demonstrate that Windows is capable of running any enterprise application, which has resulted in consistent price/performance records (see the TPC) and significant acceptance in the enterprise market.

The most widely used version of the Microsoft Windows family is Windows XP, released on October 25, 2001.

In November 2006, after more than five years of development work, Microsoft released Windows Vista, a major new operating system version of Microsoft Windows family which contains a large number of new features and architectural changes. Chief amongst these are a new user interface and visual style called Windows Aero, a number of new security features such as User Account Control, and a few new multimedia applications such as Windows DVD Maker. A server variant based on the same kernel, Windows Server 2008, was released in early 2008.

Windows 7 is currently under development; Microsoft has stated that it intends to scope its development to a three-year timeline, placing its release sometime at the end of 2009.

Unix and Unix-like operating systems

Ken Thompson wrote B, mainly based on BCPL, which he used to write Unix, based on his experience in the MULTICS project. B was replaced by C, and Unix developed into a large, complex family of inter-related operating systems which have been influential in every modern operating system (see History).

The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "UNIX" is a trademark of The Open Group which licenses it for use with any operating system that has been shown to conform to their definitions. "Unix-like" is commonly used to refer to the large set of operating systems which resemble the original Unix.

Unix-like systems run on a wide variety of machine architectures. They are used heavily for servers in business, as well as workstations in academic and engineering environments. Free software Unix variants, such as GNU, Linux and BSD, are popular in these areas.

Market share statistics for freely available operating systems are usually inaccurate since most free operating systems are not purchased, making usage under-represented. On the other hand, market share statistics based on total downloads of free operating systems are often inflated, as there is no economic disincentive to acquire multiple operating systems so users can download multiple systems, test them, and decide which they like best.

Some Unix variants like HP's HP-UX and IBM's AIX are designed to run only on that vendor's hardware. Others, such as Solaris, can run on multiple types of hardware, including x86 servers and PCs. Apple's Mac OS X, a hybrid kernel-based BSD variant derived from NeXTSTEP, Mach, and FreeBSD, has replaced Apple's earlier (non-Unix) Mac OS.

Unix interoperability was sought by establishing the POSIX standard. The POSIX standard can be applied to any operating system, although it was originally created for various Unix variants.

Mac OS X

Mac OS X is a line of proprietary, graphical operating systems developed, marketed, and sold by Apple Inc., the latest of which is pre-loaded on all currently shipping Macintosh computers. Mac OS X is the successor to the original Mac OS, which had been Apple's primary operating system since 1984. Unlike its predecessor, Mac OS X is a UNIX operating system built on technology that had been developed at NeXT through the second half of the 1980s and up until Apple purchased the company in early 1997.

The operating system was first released in 1999 as Mac OS X Server 1.0, with a desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, five more distinct "end-user" and "server" editions of Mac OS X have been released, the most recent being Mac OS X v10.5, which was first made available in October 2007. Releases of Mac OS X are named after big cats; Mac OS X v10.5 is usually referred to by Apple and users as "Leopard".

The server edition, Mac OS X Server, is architecturally identical to its desktop counterpart but usually runs on Apple's line of Macintosh server hardware. Mac OS X Server includes work group management and administration software tools that provide simplified access to key network services, including a mail transfer agent, a Samba server, an LDAP server, a domain name server, and others.

What is an operating system?

An operating system is a layer of software which takes care of technical aspects of a computer's operation. It shields the user of the machine from the low-level details of the machine's operation and provides frequently needed facilities. There is no universal definition of what an operating system consists of. You can think of it as being the software which is already installed on a machine, before you add anything of your own. Normally the operating system has a number of key elements: (i) a technical layer of software for driving the hardware of the computer, like disk drives, the keyboard and the screen; (ii) a filesystem which provides a way of organizing files logically, and (iii) a simple command language which enables users to run their own programs and to manipulate their files in a simple way. Some operating systems also provide text editors, compilers, debuggers and a variety of other tools. Since the operating system (OS) is in charge of a computer, all requests to use its resources and devices need to go through the OS. An OS therefore provides (iv) legal entry points into its code for performing basic operations like writing to devices.

Operating systems may be classified by both how many tasks they can perform `simultaneously' and by how many users can be using the system `simultaneously'. That is: single-user or multi-user and single-task or multi-tasking. A multi-user system must clearly be multi-tasking. The table below shows some examples.

OS	Users	Tasks	Processors
MS/PC DOS	S	S	1
Windows 3x	S	QM	1
Macintosh System 7.*	S	QM	1
Windows 9x	S	M*	1
AmigaDOS	S	M	1
hline MTS	M	M	1
UNIX	M	M
VMS	M	M	1
NT	S/M	M
Windows 2000	M	M
BeOS (Hamlet?)	S	M

The first of these (MS/PC DOS/Windows 3x) are single user, single-task systems which build on a ROM based library of basic functions called the BIOS. These are system calls which write to the screen or to disk etc. Although all the operating systems can service interrupts, and therefore simulate the appearance of multitasking in some situations, the older PC environments cannot be thought of as a multi-tasking systems in any sense. Only a single user application could be open at any time. Windows 95 replaced the old coroutine approach of quasi-multitasking with a true context switching approach, but only a single user system, without proper memory protection. Windows NT added a proper kernel with memory protection, based on the VMS system, originally written for the DEC/Vax. Later versions of Windows NT and Windows 2000 (a security and kernel enhanced version of NT) allow multiple logins also through a terminal server. Windows 2000 thus has comparable functionality to Unix in this respect.

The Macintosh system 7 can be classified as single-user quasi-multitasking^1.1. That means that it is possible to use several user applications simultaneously. A window manager can simulate the appearance of several programs running simultaneously, but this relies on each program obeying specific rules in order to achieve the illusion. The MacIntosh not a true multitasking system in the sense that, if one program crashes, the whole system crashes. Windows is purported to be preemptive multitasking but most program crashes also crash the entire system. This might be due to the lack of proper memory protection. The claim is somewhat confusing.

AmigaDOS is an operating system for the Commodore Amiga computer. It is based on the UNIX model and is a fully multi-tasking, single-user system. Several programs may be actively running at any time. The operating system includes a window environment which means that each independent program has a `screen' of its own and does not therefore have to compete for the screen with other programs. This has been a major limitation on multi-tasking operating systems in the past.

MTS (Michigan timesharing system) was the first time-sharing multi-user system^1.2. It supports only simple single-screen terminal based input/output and has no hierarchical file system.

Unix is arguably the most important operating system today, and one which we shall frequently refer to below. It comes in many forms, developed by different manufacturers. Originally designed at AT&T, UNIX split into two camps early on: BSD (Berkeley software distribution) and system 5 (AT&T license). The BSD version was developed as a research project at the university of Berkeley, California. Many of the networking and user-friendly features originate from these modifications. With time these two versions have been merged back together and most systems are now a mixture of both worlds. Historically BSD Unix has been most prevalent in universities, while system 5 has been dominant in business environments. The trend during the last three years by Sun Microsystems and Hewlett-Packard amongst others has been to move towards system 5, keeping only the most important features of the BSD system. A standardization committee for Unix called POSIX, formed by the major vendors, attempts to bring compatibility to the Unix world. Here are some common versions of UNIX.

Unix	Manufacturer	Mainly BSD / Sys 5
BSD	Berkeley	BSD
SunOS (solaris 1)	Sun Microsystems	BSD/sys 5
Solaris 2	Sun Microsystems	Sys 5
Ultrix	DEC/Compaq	BSD
OSF 1/Digital Unix	DEC/Compaq	BSD/sys 5
HPUX	Hewlett-Packard	Sys 5
AIX	IBM	Sys 5 / BSD
IRIX	Silicon Graphics	Sys 5
GNU/Linux	Public Domain	Posix (Sys V/BSD)
SCO unix	Novell	Sys 5

Note that the original BSD source code is now in the public domain. Unix is generally regarded as the most portable and powerful operating system available today by impartial judges, but NT is improving quickly. Unix runs on everything from laptop computers to CRAY mainframes. It is particularly good at managing large database applications and can run on systems with hundreds of processors. Most Unix types support symmetric multithreaded processing and all support simultaneous logins by multiple users.

NT is a `new' operating system from Microsoft based on the old VAX/VMS kernel from the Digital Equipment Corporation (VMS's inventor moved to Microsoft) and the Windows32 API. Initially it reinvented many existing systems, but it is gradually being forced to adopt many open standards from the Unix world. It is fully multitasking, and can support multiple users (but only one at a time-- multiple logins by different users is not possible). It has virtual memory and multithreaded support for several processors. NT has a built in object model and security framework which is amongst the most modern in use.

The Be operating system, originally developed for a new multimedia computer called the BeBox, is also new and is a fully multitasking OS. It is optimized for multimedia and is now saleable software developed by Be.Com after the new computer concept failed due to lack of financial backing. BeOS has proper memory protection but allows direct access to video memory (required for fast video games). It also has virtual memory, is pre-emptive multitasking and is based on a microkernel design. Is shares little with Unix except for a Bash shell, a POSIX programming interface and about 150 Unix commands (including Perl).