Early operating systems were known as Machine Control Programs (MCP) or Disc Operating System (DOS). Software on a computer system is split into two main groups known as Application Software and System Software. System Software is split up into Utility Software and the OS.

The OS consists of the following:

• The Shell (command) is the user interface
• The Kernel runs the programs
• A windows system (if used)

The BIOS changes dependent on the hardware. BIOS stands for "Basic Input/Output System". Its main job is to load and execute the Operating System normally from the computer's hard disk. Drivers are pieces of code added to the BIOS so that new pieces of hardware can be used properly.

The Resource manager controls the machine and schedules activities. It carries out time management i.e. shares out CPU time. It allocates memory space to programs and disk space to files as well as maintaining the folder structure the files are stored in. File managers (such as DOS) are a component of the OS that manage where files are located in the system's mass storage. They also manage how these files are accessed. A Database Management System (DBMS) manages any databases on the computer.

Multi-user systems allow multiple people to use the system either at the same time or sequentially. Involved in these systems are functions that keep the users' activities and data private. Multi-tasking involves the running of several different programs at the same time. This is the main function of the kernel.

Paged memory management is a way of organising applications is memory. Parts of each application are scattered across memory. This means that however large an application is, it should always be able to fit in because it does not need a large single block of memory to reside in. This technique also removes the necessity for applications to be moved around in memory in order to fit in larger applications. Problems would otherwise occur when devices using DMA try to communicate with these applications and they have moved in memory.

Memory is used effectively by paged memory management because code that is loaded once can be re-used by a different application. These small bits of code are known as DLLs.

Context switches are where the current application being executed is switched. This can be due to interupts that invoke a change in state (privileged or user state).

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