Introduction to Operating Systems

A short introduction to operating systems

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).