An operating system is a set of programs that manages the operation of a computer.

Operating systems have become increasingly large and complex with the development of computer hardware and the use of multi-user systems.

The most frequently used sections of the operating system must be loaded into memory and stay there while applications are being run. When a computer is first switched on, the OS must be transferred from disc to memory. This process is called 'booting' the system.

There has to be a small program held in ROM which initiates the booting process.

The main functions of an Operating System.

• Resource management
  All operating systems must be able to load programs from backing storage devices, store them in memory and start them running; more complex operating systems have to allocate CPU time, memory and I/O resources to different programs.
  
• Memory management
  Where more than one process is to be run, the operating system has to allocate sections of memory (partitions) to each, and to make sure that one process does not use the memory allocated to another...or else...crash!!! Inactive partitions may be swapped on to backing store.
  
• Backing store management
  Including the creation, updating and deletion of files. Managing the FAT (File Allocation Table) and the directory of files.
  
• I/O Control
  Controlling data flow to and from peripherals.
  
• Interrupt handling
  Detecting and servicing interrupts.
  
• Operator interface
  Receiving and processing instructions from the operator. Reporting errors.
  
• Security
  Maintaining logging in codes and passwords; keeping a log of each user's time on the system.
  

Most operating systems also have utility programs for file management, editing files, reorganising disc space, making backups, copying files,  etc...