Operating System #06 Virtual Memory & Demand Paging in Operating Systems

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Operating System #06 Virtual Memory & Demand Paging in Operating Systems
Operating System #06 Virtual Memory & Demand Paging in Operating Systems
Operating System #06 Virtual Memory & Demand Paging in Operating Systems
Complete Operating Systems Lecture/ Tutorials from IIT @ https://goo.gl/GMr3if
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00:30 Virtual Memory: In virtual memorty RAM is split into fixed size partitions called page frames. Page frames typically 4KBs. Process splits into block of equal size. Here block size is equal to page frame size.Because of the page table, blocks need not to be in contiguous page frames. Every time a memory location is accessed, the processor looks into the page table to identify the corresponding page frame number. In virtual memory model, blocks from several processes can share pages in RAM simultaneously.
11:42 Demand Paging: In demand paging, pages are loaded from disk to RAM, only when needed. A present bit in the page table indicates if the block is in RAM or not. If a page is accessed that is not present in RAM, the processor issues a page fault interrupt, triggering the OS to load the page into RAM and mark the present bit to 1. If there are no page free for a new block to be loaded the OS makes a decision to remove another block from RAM which is based on a replacement policy implemented in OS. Some replacement policies are First IN First Out, Least Recently USED, Least Frequently Used. The replaced block may need to be written back to the swap. The dirty bit in the page table indicates if a page needs to be written back to disk. Protection bits in the page table determines if the page is executable, readonly and accessible by a user process.

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