Allocation Of Frames In Operating System

In this post we will cover the topic :- allocation of frame. As we know an important aspects of operating system is virtual memory, which is implemented by using demand paging technique. Demand paging necessitates the development of a page replacement algorithm and a frame allocation algorithm.

Frame allocation algorithms are used if you have multiple processes. It helps to decide how many frames are allocated to each process. Consider a single user system with 128 KB of memory composed of 1 KB in size. This system has 128 frames. The operating system may take 35 KB leaving 93 frames for the user processes.

Under pure demand paging all 93 frames would initially be put on free frame list. When a user process started execution, it would generate a sequence of page faults. The first 93 page faults would all get free frames from the free frame list. When the free frame list exhausts, a page replacement algorithm would be used to select one of the 93 in memory pages to be replaced with 94th and so on. When the process is terminated, the 93 frames would once again be placed in free frame list.

There are various constraints to the stratigies for the allocation of frames :-

One can not allocate more than the total number of available frames.

At least a minimum number of frames should be allocated to each process.

This constraints are supported by two reasons :-

1. As less number of frames are allocated, there is an incresase in the page fault ratio, decreasing the performance of execution of the process.

2. There should be enough frames to hold all the different pages that any single instruction can reference.

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