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Allocating System Resources





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System Software: Functions

All operating systems must handle the same basic tasks. These functions can be divided into three groups:

 

The operating system directs the traffic inside the computer, deciding what resources will be used and for how long.

Time Time in the CPU is divided into time slices which are measured in milliseconds. Each task the CPU does is assigned a certain number of time slices. When time expires, another task gets a turn. The first task must wait until it has another turn. Since time slices are so small, you usually can't tell that any sharing is going on. Tasks can be assigned priorities so that high priority (foreground) tasks get more time slices than low priority (background) tasks.
Memory Memory must be managed also by the operating system. All those rotating turns of CPU use leave data waiting around in buffers. Care must be taken not to lose data!! One way to help out the traffic jam is to use virtual memory. This includes disk space as part of main memory. While it is slower to put data on a hard disk, it increases the amount of data that can be held in memory at one time. When the memory chips get full, some of the data is paged out to the hard disk. This is called swapping. Windows uses a swap file for this purpose.
Input and output Flow control is also part of the operating system's responsibilities. The operating system must manage all requests to read data from disks or tape and all writes to these and to printers. When you click the mouse while the web cam is streaming, the operating system must control what happens and when. To speed up the output to printers, operating systems now allow for print spooling, where the data to be printed is first put in a file. This frees up the processor for other work in between the times data is going to the printer. A printer can only handle so much data at a time. Without print spooling you'd have to wait for a print job to finish before you can do anything else. With it you can request several print jobs and go on working. The print spool will hold all the orders and process them in turn.
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