Discussion
In paging, the page table is used to
Answer: A
Correct answer: translate a logical page number into a physical frame number
A page table maps each logical page of a process to the physical frame holding it. A logical address is split into a page number and an offset; the page number indexes the table to give a frame number, and the offset is carried across unchanged. That indirection is what makes paging work. A process’s pages need not be contiguous in physical memory, so external fragmentation disappears, and each process gets its own address space isolated from every other. Its cost is an extra memory access for every reference, which would halve performance. The remedy is the translation lookaside buffer, a small associative cache of recent translations; hit rates above 90 per cent bring the average close to a single access. Each entry also carries protection and status bits - valid or invalid, read-only, and a modified or dirty bit recording whether the page must be written back when replaced. For large address spaces the table is itself paged, giving multilevel page tables.
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