Discussion
The 50-per-cent rule in memory allocation states that
Answer: D
Correct answer: for every two allocated blocks, about one block is lost to fragmentation
The 50-per-cent rule is a statistical result about first-fit allocation: given \( N \) allocated blocks, about \( 0.5N \) blocks are lost to external fragmentation, so roughly a third of memory may be unusable. It is derived from the observation that in a steady state, requests and releases interleave in a way that leaves a hole beside about half the allocated blocks. The rule is empirical rather than exact, but it is a good guide to what contiguous allocation costs. Its practical significance is that it quantifies why paging is preferred. A third of memory unusable is a heavy price, and paging removes external fragmentation entirely, since any free frame will serve for any page. The remaining waste under paging is internal fragmentation of at most one page per process - on average half a page, which for a 4 KB page is 2 KB. Against a third of memory, that is negligible. Compaction is the alternative cure, but it costs a great deal of copying.
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