What is an Operating System?

An Operating System is a PROGRAM that acts as an interface between a USER of a computer system and the COMPUTER HARDWARE.
OS manages the computer hardware, and provides a basis for application programs. There is no universally accepted definition of OS and what is part of OS and what is not; Is Internet Explorer a part of Windows OS?
Kernel – a program that is running all the time on the computer. Other programs are considered as system/application programs.














User views of OS

• PC: single user ->OS provides ease of use, less attention to performance, and (almost) none to resource utilization.
• Mainframe or minicomputer: many users share resources and exchange information -> OS maximizes resource utilization/efficiency, and fairness among users.
• Workstations: several users with both shared and private resources -> OS to compromise between individual usability and resource utilization.
• Handheld computers: single user with power and interface limitations -> OS aims for individual usability and performance per battery life.
• Embedded computers: have little or no user view OS-> runs without user intervention.

System views of OS

• OS as the resource allocator OS must use the resources efficiently and allocate them fairly among users. Resources: CPU time, memory space, file storage space, I/O devices, etc.
• OS as the control program Control the execution of user programs OS prevents errors and improper use of the computer.

OS Purposes

1) To provide an environment for a user to execute programs conveniently and efficiently.

2) To simulate features not available on hardware.
OS provides features factually not provided by the hardware.
What features?
3) To control all the computer’s resources and to provide the base upon which the application programs can be written.