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, 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 program is loaded, the operating system has to allocate sections of memory (partitions) to each program.
  
• 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...