ITPEC IP April 2021 Question 17

To run a program that needs more memory than the main memory has — virtual memory uses auxiliary storage as an extension of main memory.

The OS gives each program a large virtual address space and divides it into pages. Only the pages in use are kept in main memory; the rest wait on auxiliary storage such as a disk and are swapped in when needed (paging). A program larger than main memory can therefore run, at the cost of slower access when pages move.

Answer (c)

Why not others:

• •(a) — keeping programs out of the OS's area of main memory is memory protection, a separate OS function
• •(b) — writing memory to storage and cutting the power while idle is hibernation, a power-saving function
• •(d) — a storage device faster than main memory that speeds up the CPU is cache memory; virtual memory relies on slower auxiliary storage

Key rule: Virtual memory trades speed for capacity: it lets programs use more memory than is installed, while cache memory trades capacity for speed.

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