Discussion
Virtual memory allows a computer to
Answer: D
Correct answer: execute programs larger than the available physical memory
Virtual memory lets a process run without all of it being resident in physical memory. Pages are brought in from disk as they are referenced - demand paging - and pages not currently needed are held on the backing store. A reference to a page that is not resident raises a page fault, a trap the operating system services by finding a free frame, reading the page in, updating the page table and restarting the instruction. Three benefits follow. Programs may be larger than physical memory. More processes fit in memory at once, so CPU utilisation and throughput rise. And less input and output is needed to load a program, since only the parts used are read. The cost is that a page fault takes milliseconds against nanoseconds for a memory access, so the fault rate must stay very low - and if it does not, the system thrashes. It also depends on locality of reference: without it, demand paging would fault constantly.
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