Operating Systems

Apart from the code in the Basic Input/Output System (BIOS), the operating system is the first programming that runs when you power up a computer.

The operating system (OS) is the fundamental system software that controls everything the computer does.

Operating System Architecture, thanks to How Stuff Works

It:

- provides a standard and predictable interface for users
- provides a standard and predictable platform for applications
- controls the operation of the CPU
- allocates memory to programs, and protects tasks from interfering with each other's memory
- controls running applications and manages their multitasking
- negotiates with and controls peripheral controllers such as hard disks, video cards, USB devices through their device drivers
- manages disk storage and retrieval
- runs background tasks necessary for the computer and its services (e.g. internet access) to work
- oversees security such as logging in, protecting one user's settings from other users
- checks for security updates from the manufacturer, or driver updates from manufacturers of connected devices

Most modern operating systems are GUI - Graphical User Interface - where the user controls the computer's underlying services using the metaphor of a desktop where objects such as storage devices, applications and services are represented by icons that can be clicked or dragged to trigger an event in the OS. GUI is also sometimes called WIMP (windows, icons, mouse pointer).

Earlier OSs used a text-based command line interface (CLI) where commands were typed as words with parameters and flags used to give the command more information.

e.g. in DOS, the command DEL ..\*.bat /s /q would delete (DEL) all files with the extension ".bat" (*.bat) in the parent directory (..\) and subdirectories (/s) without asking for permission (/q). CLI is powerful and flexible, but requires much study and practice to master.

Some (most?) GUI OSs (e.g. Windows, Linux) have an underlying CLI that can be used when a GUI just is not powerful enough. Some things are still best done the old fashioned way!

GUI was a revolution because it was so easy for beginners to use, which made personal computer ownership skyrocket.

• Windows (current version is 'XP', soon to be 'Vista') - about 90% of the desktop OS market
• Apple OS (current version X) - about 10% of the desktop OS market
• Linux (open source version of UNIX - high market share in web servers)

Lesser-known varieties include Amiga OS, Palm OS which controls the Palm computers and mobile phones.

Did you know that many mobile phones, routers and other devices use Linux as their operating system?

Main OS Types

• Single User, Single Task - e.g. DOS, Palm OS. Only 1 user at a time running 1 program at a time.
• Single User, Multitasking - e.g. Windows, Mac OS. One user at a time can run several programs simultaneously*
  
• Multiuser OS - e.g. mainframes, UNIX. Several people can use the same computer at the same time. This is different to (for example) a Windows desktop PC with different user accounts for mum, dad and the kids... only one of them can use the computer at a time!

* 'Multitasking' on PCs is really a fraud. Only one application is actually running at any single moment, but the CPU is flicking so rapidly giving time to each task in turn that it appears to be simultaneous... just as a 35mm film's individual frames give the illusion of motion when shown in rapid sequence. Multicore CPUs will start to give REAL multitasking!