Operating Systems

There are several different types of operating systems:

Single User operating systems allow only one user to make use of a PC at any given time. Single User operating systems are common on personal computers, eg: Windows 7, Apple Mac OS X.

Multi-User operating systems were originally used on mainframes, large and expensive computers used mainly by government departments and large companies for high-volume data processing tasks such as censuses, industry/consumer statistics and financial transaction processing. Nowadays they are often used on networked systems. There are several different types of multi-user system:

  • Single Processor systems have only a single processor or CPU, which is shared between users by dividing the CPU time into time-slices and allocating one of these to each user in turn. The time-slices are very short, giving each user the impression that their programs are running continuously.
  • Multiple Processor systems have more than one processor. Users still have to share processors, but performance is improved as there are fewer users per processor. Some supercomputers have thousands of processors running in parallel.
  • Networked Systems consist of single user PCs connected together to form a Local Area Network (LAN). Each PC has a Network Interface Card (NIC) and they are normally connected by means of copper or fibre cables, although wireless networks are becoming increasingly common. Networks allow users to share files and resources such as a printer or an Internet connection. Common network operating systems include Windows NT, Windows 2000/2003 Server and Unix/Linux.
  • Multi-Tasking Systems are those which can carry out several tasks simultaneously, eg: printing a document while carrying out calculations within a spreadsheet. Multi-tasking systems can be single user or multi-user and they can use either pre-emptive multi-tasking, where the OS handles the switching of resources between tasks, or co-operative multi-tasking, where each task relinquishes control at an appropriate stage, eg: while awaiting input from a user.

Real Time Systems are those used for direct control over electromechanical equipment such as industrial robots, factory processes, power stations, airplanes, trains and cars. They monitor external events and react instantaneously, hence the expression “real time”.

Distributed systems consist of multiple autonomous computers that communicate through a computer network and interact with each other in order to achieve a common goal. The term “distributed system” originally referred to computer networks where individual computers were physically distributed within some geographical area. It is used nowadays in a much wider sense, even when referring to independent processes that run on the same physical computer and interact with each other by message passing.

Next: Operating System Layers