The operating system is the first software we see once the computer has booted and the last software seen when we shutdown. 

It organises and controls the hardware and other software used by the system. 

Operating systems provide a stable and consistent way for applications to use hardware without them having to know the precise nature of the hardware.

Types of Operating Systems

• Real Time OS - controls machinery and scientific instructments

• Single User, Single Task OS - e.g. Symbian

• Single User, multi-tasking OS - eg. Windows, Apple OSX etc

• Multi User OS - allows access to system resources by many users, protecting each user from other users - each user thinks they have the machine to themselves..  E.g. Mainframes - IBM Multiple Virtual Storage (MVS) 

Functions of Operating Systems

• Processor management - allocates processes to processors, taking into account delays from input/output devices

• Memory and storage management - each process is allocated sufficient memory to execute

• Device Management - communicating with Drivers - which translate messages from computer to that which can be understood by device

• Application Interface - allows application software to communicate with OS functions.

• User Interface - provides a mechanism for User to interact with the Computer OS functions - MS DOS prompt, GUI interfaces like Windows.  These sit on top of the main operating system

Utilities

• File Compression

• Defragmentation

• Virus Checking

• Embedded Licence installation count

• Batch Job Scheduling - e.g. Auto Virus Checking & Defragmentation

• Emulation - emulate other OS - e.g. GUI shell on Linux or MS Windows on MAC