On an average 50 of the memory may remain unused due

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Unformatted text preview: available for use by the currently active user process. In this scheme, the operating system loads a program to be executed from disk into the user area of the memory and executes it. When the process finishes, the operating system cleans up the user area of the memory and then loads the next program to be executed. Although this memory management scheme is simple and easy to implement, it does not lead to proper utilization of the available main memory resource. This is because whatever memory space is not occupied by the currently active user process remains unusued for the entire duration of the process's execution. Hence this memory management scheme is now used only on very small or dedicated computer systems. Multiprogramming Memory Models We saw that in a multiprogramming system multiple user processes need to simultaneously reside in the main memory. The two memory management schemes used to facilitate this are multiprogramming with fixed number of memory partitions and multiprogramming with variable number of memory partitions. They are described below. Multiprogramming with Fixed Number of Memory Partitions In this scheme, the user area of the memory is divided into a number of fixed-sized partitions. The partitions may be of equal or unequal size, but the size, of each partition is fixed. Figure 14.12 shows a multiprogramming memory model with n equal-sized partitions. Each partition may contain exactly one process. Thus, the degree of multiprogramming is bounded by the number of partitions. That is, at a time only n processes can be loaded in the system. All new jobs are put into an input queue. When a partition is free, the next job from the input queue is picked up and loaded into the free partition. When a process terminates, the partition occupied by it becomes free for use by another process waiting in the input queue. Note that if the system uses unequal-sized partitions, when a partition becomes free, the process closest to the front of the input queue that fits in it could b...
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This document was uploaded on 04/07/2014.

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