A modern general-purpose C/S consists of a CPU and a number of device controllers that are connected through a common bus that provide access to shared memory. See figure (2.1).
Figure (2.1) A modern computer system

Operating systems is a program that acts as an intermediary between a
user of a computer and the computer hardware (H/W). The purpose of an
operating system is to provide the environment in which the user can execute
programs. The O.S. is a resource allocator (managers all resources, decides
between conflicting requests for efficient and fair resource use) and is a
control program (controls execution of programs ) to prevent errors and
improper use of the computer.
1.2 Computer System Components:

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