10 Virtual Memory

Only the required page of a process is required to be brought to physical memory; when a page is not required for execution, it is not brought in

Main usecases¶

Rarely used features/functions and data structures

Advantages¶

• Size of process is not limited by the size of physical memory (primary)
• Increased degree of multiprogramming (no of programs that can exist in the primary memory at the same time)
• Increased CPU utilization
• Reduced I/O wrt a process, by eliminating unnecessary ‘swapping-in’

Page Fault¶

When a process tries to access a page that exists in memory, execution continues as normal

Otherwise, if the process tries to access a page that is marked invalid, this means that the corresponding page is missing

• software interrupt (trap) is created
• bring in required page from secondary memory
• Store into free frame/Page Replacement
• Reset page table
• Restart execution

Page Replacement¶

If there are no free frames, we need to replace the frame in a manner that would reduce future page faults.

Algorithms¶

Bélády's Anomaly¶

In computer storage, the phenomenon in which having more page frames can cause more page faults for first-in first-out page replacement algorithm

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