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System Software

Notes When a computer has multiple users, the needs for managing and protecting the memory,
input/output devices and other resources are vary necessary. These needs arises because it is
usually necessary to share expensive resource such as tapes drives and phototypesetters.
Operating systems perform basic tasks, such as recognizing input from the keyboard, sending
output to the display screen, keeping track of files and directories on the disk, and controlling
peripheral devices such as disk drives and printers.
For large systems, the operating system has even greater responsibilities and powers. It is like
a traffic cop -- it makes sure that different program and users running at the same time do not
interfere with each other. The operating system is also responsible for security, ensuring that
unauthorized users do not access the system.
An operating system performs the following services for applications:
 In a multitasking system where multiple programs are run at the same time, the operating
system determines which applications should run in what order and how much time
should be allowed for each application before another application gets its turn.
 It sends messages to each application or interactive user (or to a system operator) about
the status of operation and any errors that may have occurred.
 It handles input and output to and from attached hardware devices, such as hard disks,
printers, and dial-up ports.
 It arranges the sharing of internal memory among multiple applications.
 It can offload the management of what are called batch jobs (for example, printing) so that
the initiating application is freed from this work.
 In computers that can provide parallel processing, an operating system manages to divide
the instructions of a program so that it runs on more than one processor at a time.

2.3.1 Other Operating System Functions

Figure 2.12: Interface of an Operating System

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