Operating Systems



Operating Systems

What is an Operating System?

A computer is essentially a machine that can be programmed to carry out a series of calculations in a given order.

Early computers ran programs that would each carry out a specific task.  Each program would be written from scratch to meet the requirements of that task.

As single computers were used to run a wider variety of programs, it soon became apparent that many of these programs shared common features — such as control of peripherals or standard mathematical functions.  Programmers realised that a common series of functions, made permanently available to all programs, would be a great asset and would speed the process of writing software.  Thus the operating system was born.

Operating systems of the 1960s and 1970s provided features to control electronics, process keyboard input and write text to display screens.  More advanced OSes could organise data into files on tape and disk drives, and even manage the execution of other programs.

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In the 1980s and 1990s great advances were made in the field of user interface, and graphical shells were added to make computers easier to use.

An operating system controls the computer electronics, provides programs with common subroutine functions, manages data storage and provides a consistent interface to allow users to operate the computer.

Terminology

Multitasking

Multitasking means that a computer appears to carry out more than one program simultaneously.  This is usually achieved by allocating slots of processor time to programs in turn, rotating the slots so quickly that the programs appear to be running concurrently.

Unix and Windows 95 are examples of multitasking OSes.

By contrast, MS-DOS and Windows 3.1 are "single tasking" OSes.

Multi-user

This is when many users are using the same computer.  This is typified by multiple displays and keyboards interfaced to a central machine.  By rotating attention to each user in quick succession, and using multitasking, each user gains the impression that they are the only person using the computer.

Unix and Windows 2000 Server are examples of multi-user OSes.

By contrast, MS-DOS and Windows XP are designed for single users only.

Multi-processing

More advanced OSes are able to spread processing between multiple CPUs.  This is a complex task and involves programs being broken into threads.  This type of design is also available on a larger scale on networks, where it is known as grid computing.

Unix, Windows NT and Windows XP are examples of multi-processing OSes.

By contrast, MS-DOS and Windows 98 can only run on a single processor.

Command Line Interface (CLI)

With a CLI, the computer is controlled by text instructions typed at a keyboard.  The operating system's command shell interprets the typed instructions to launch programs or carry out OS processes.  CLIs are often confusing and hard to use for many people.

Unix and MS-DOS are examples of OSes based around command-line interfaces.

Graphical User Interface (GUI)

In 1973 researchers are Xerox's Palo Alto research labs developed the WIMP system for controlling computers: Window, Icon, Mouse and Pointer.  This graphical interface was much easier for novices and revolutionised the operation of computers but didn't really take off until the mid 1980s.

Windows, Mac OS and Unix's X-Window shell are examples of OSes featuring GUIs.

Early Operating Systems

The first popular OS was IBM's OS/360 program, common to a range of that company's mainframes in the 1960s.  It slowly evolved into the modern z/OS that runs on the latest IBM equipment.

The minicomputers of the late 1960s also required operating systems, and AT&T developed an operating system that could be easily transported to different kinds of computer equipment.  This OS was called Unix and it became very popular indeed.

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Unix was so successful that it spawned a number of clones, including Posix, AIX, Ultrix, HP-UX, Xenix, FreeBSD, Solaris, A/UX and (most famously) Linux.

By the late 1970s microcomputers also gained a common operating system: CP/M.  CP/M was available for a variety of machines based around the 8080 and Z80 microprocessors and gave an early common platform to business applications programs such as the WordStar wordprocessor.

CP/M was an example of an OS that came in two parts: hardware-specific code (Basic Input/Output System) — stored in a ROM chip — and general OS code with support for disk drives (Disk Operating Systems) — usually stored on floppy or hard disk.

Although CP/M gave a common platform for programs the floppy disk formatting varied between BIOSes.  Programs and data were ported between different computer models via serial cables.

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CP/M was still being used into the late 1980s on non-PC microcomputers.

PC operating systems

MS-DOS

The first PC OS was the Disk Operating System written by Microsoft: MS-DOS.

MS-DOS is a single-tasking, single-user operating system with a command-line interface.  It was used primarily from 1980 until the mid-'90s.

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Technicians still use it for network login scripts, and system preparation & repair (through command-line tools like Windows PE and Recovery Console).

The final incarnation of MS-DOS was version 6.22.  Windows 9x incorporates version 7 of MS-DOS, but later versions of Windows emulate MS-DOS's command-line interface, albeit with a restricted set of commands.

MS-DOS was so successful that other manufacturers created clones of it: IBM's PC-DOS and Digital's DR-DOS.  Thus they are generically referred-to as "DOS".

Files are stored in a root directory.  This directory can contain subdirectories of files or further subdirectories.  To differentiate it from the similar Unix directory structure, using slashes ('/') between directory names, Microsoft chose to use a backslash ('\').  DOS also owes some heritage to CP/M, which is why filenames are restricted to 8 characters and a 3-character extension code.  Files are organised according to Microsoft's FAT (File Allocation Table) filing system.

Windows v1

When Apple's Macintosh computer was released in 1983 its graphical interface was certainly a factor in its popularity.  Microsoft had also been developing their own GUI and released Windows version 1 commercially in 1985.

Windows sat as a shell on top of MS-DOS, allowing specially-written programs to utilise a rudimentary system of menus, icons and windows.

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Windows was still single-tasking but gave the impression of running several programs at the same time.

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Windows 3.1

By 1990 Microsoft were up to version 3 of Windows.  This still ran as a shell on top of MS-DOS but had become the preferred interface for the PC.

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Windows 3.1 required a minimum of MS-DOS 3.1, a 386 processor with 2MB of RAM, and 6MB of hard disk space.

OS/2

IBM's OS/2 operating system had started as a command-line OS like MS-DOS but the later versions, "OS/2 Warp", featured a GUI.  Unlike DOS or Windows 3.1 this multitasked and was very stable.  OS/2 is still used in ATM cashpoints worldwide.

Windows 95

Windows 95 (Windows version 4) introduced multitasking to the PC.  The GUI was improved and DOS was now embedded into Windows.  It also had built-in networking.

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Minimum specification was a 386DX / 4MB RAM / 24MB HDD but P75 / 16MB RAM / 90MB HDD was preferred.  Major revisions were 95b (with bug fixes) and 95c (USB support).

Windows NT4

Windows NT (New Technology) was a massive resign bringing greater security, stability and multiprocessor support (but less compatibility for games / older software).  Version 3.1 was released in 1993 but it wasn't until version 4 in 1996 that this really took off.

Aimed at corporate customers, it came in two versions, NT Workstation and NT Server and required a 33MHz 486 / 16MB RAM / 110MB HDD.  Service packs were released at intervals, finishing at Service Pack 6a.  NT4 is still used widely.

Most importantly, NT gave support for alternative filing systems (such as IBM's Posix) and the new NT Filing System (NTFS) that allowed users to be given rights governing access to file and folders.

Novell Netware

Novell's dominance in corporate networking was challenged by NT.  Novell's networks run around servers running Netware.  Until recently, Netware had a text-only menu-driven interface which allowed for greater speed and stability.  Older versions run on a 486 / 16MB / 1GB.  The latest version (6.5) requires P200 / 512MB / 2GB.  The next version of Netware will be built around SuSE Linux.

Windows 98

Windows 98 was an enhanced version of Windows 95.  Its major features were a new filing system (FAT32) and USB and DVD support.  Internet Explorer was integrated more, allowing Windows Explorer to switch between web and local files easily.

The original FAT12 (floppy disk) and FAT16 (hard disk) systems used by MS-DOS broke files into clusters.  FAT16 only allowed up to 65536 clusters of up to 32KB size, meaning that only 2GB of disk space could be accessed.  Thus large hard disks had to be partitioned into 2GB areas.  FAT32 changed this by allowing more clusters of a smaller size, supporting disks up to 2TB size more efficiently.

Windows 98 requires a 486DX2/66 processor, 16MB RAM and 200MB disk space.

A major revision, Windows 98 SE, brought bug-fixes and Internet Connection Sharing.

Windows Me

Windows Me (Millennium Edition) was a final attempt to enhance the Windows 9x range of OSes.  Significant features included the ability to revert back to a previous setup and improved video support.  Requires P150 / 32MB RAM / 300MB HDD.

Windows 2000

Windows 2000 is NT version 5.  A significant proportion of the OS was rewritten, giving 2000 some advanced features.  It supports FAT, FAT32 and NTFS filing systems and introduces on-the-fly file encryption, and support for USB (missing from NT4).

Windows 2000 (known as Win2k) comes in three main versions: 2000 Professional, 2000 Server and 2000 Advanced Server.  Server versions add Active Directory, a system for managing network and user information in a distributed database, and Terminal Services, a remote login utility.

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2000 Pro requires a minimum of a P133 / 32MB RAM / 650MB HDD.  Service pack 4 is current.

Windows XP

Windows XP is NT version 5.1 and is aimed at both home users (XP Home) and businesses (XP Professional, with better networking support).  Two other specialised versions, XP Media Center Edition and XP Tablet PC edition are also available.

It offers improvements over Windows 2000 including enhanced user interface and a single-user version of Terminal Services called Remote Desktop (RDP).

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Minimum specification is P233 / 64MB / 1.5GB but it can run on slower systems.

Service Pack 2 is the current version.

Windows Server 2003

Windows Server 2003 is NT version 5.2, and it initially looks to be "XP Server".  However this versions has been deconstructed and rebuilt with extensive testing to emphasise security.

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Server 2003 includes features from 2000 Server (Active Directory, Terminal Services).

Minimum hardware is P133 / 128MB / 1.5GB, preferred is P500 / 256MB / 2GB.

Enterprise Edition supports 8 processors, hot-swappable RAM and 64-bit instructions.

Linux

Linux is an open source clone of Unix.  There are a number of free versions (distros) available, and some commercial versions which include specialist software.

Popular distros include Red Hat, SuSE and Mandrake.  Typical requirements are usually Pentium / 64MB / 2GB.

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Linux is cheaper than Windows but requires greater knowledge to manage and use.

Apple operating systems

The Apple Macintosh is one of the most popular alternatives to the PC.  Its early success was due to two factors: graphical user interface and Desktop Publishing (DTP) software.  Because of its prominence in the publishing industry the Mac has also become the prominent tool for graphic design and video production.

System 1 to MacOS 9

There were 9 main revisions of the Apple Macintosh operating system.

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System 1 to System 6 were similar to Windows 3.1 and could only run one program at a time.  System 7 brought multitasking and and an improved user interface, which improved further with MacOS 8 and 9.

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MacOS X

MacOS X rebuilt the operating system around a Unix kernel, giving greater stability and compatibility.

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OS X requires a G3, G4 or G5 processor, 128MB RAM and 2GB disk space.

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