What is Page Replacement in Computer Architecture?

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A virtual memory organization is a consolidation of hardware and software systems. It can make efficient utilization of memory space all the software operations are handled by the memory management software.

The hardware mapping system and the memory management software together form the structure of virtual memory.

When the program implementation starts, one or more pages are transferred into the main memory and the page table is set to denote their location. The program is implemented from the main memory just before a reference is created for a page that is not in memory. This event is defined as a page fault.

When a page fault appears, the program that is directly in execution is stopped just before the required page is transferred into the main memory. Because the act of loading a page from auxiliary memory to main memory is an I/O operation, the operating framework creates this function for the I/O processor.

In this interval, control is moved to the next program in the main memory that is waiting to be prepared in the CPU. Soon after the memory block is assigned and then moved, the suspended program can resume execution.

If the main memory is full, a new page cannot be moved in. Therefore, it is important to remove a page from a memory block to hold the new page. The decision of removing specific pages from memory is determined by the replacement algorithm.

There are two common replacement algorithms used are the first-in, first-out (FIFO) and least recently used (LRU).

The FIFO algorithm chooses to replace the page that has been in memory for the highest time. Every time a page is weighted into memory, its identification number is pushed into a FIFO stack.

FIFO will be complete whenever memory has no more null blocks. When a new page should be loaded, the page least currently transports in is removed. The page to be removed is simply determined because its identification number is at the high of the FIFO stack.

The FIFO replacement policy has the benefit of being simple to execute. It has the drawback that under specific circumstances pages are removed and loaded from memory too frequently.

The LRU policy is more complex to execute but has been more interesting on the presumption that the least recently used page is an excellent applicant for removal than the least recently loaded page as in FIFO. The LRU algorithm can be executed by relating a counter with each page that is in the main memory.

When a page is referenced, its associated counter is set to zero. At permanent intervals of time, the counters related to all pages directly in memory are incremented by 1.

The least recently used page is the page with the largest count. The counters are known as aging registers, as their count denotes their age, that is, how long ago their related pages have been referenced.

Published on 27-Jul-2021 14:30:46