Memory Management Questions and Answers

Answer: C

Correct answer: both (a) and (b)

A data type is defined by how its values are stored - how many bytes, in what representation - and by what may be done with them. Integers and reals differ in both respects, which is why the two together define the type.

Answer: A

Correct answer: the cost incurred

Compaction works, but every byte of every process must be copied, and nothing else can run while it happens. That expense is its drawback.

Answer: C

Correct answer: read – write

The protection bit records whether the segment may be written as well as read, so read-only segments - shared code, constants - can be protected from modification.

Answer: B

Correct answer: a two level paging algorithm

A large address space would need a page table too big to hold in memory as one piece, so the table itself is paged: an outer table indexes pages of the inner table. Only the parts actually used need be resident.

Answer: C

Correct answer: is in the process's logical address space

The valid bit says the page is part of this process's address space; invalid means the page number lies outside it, and a reference to it is trapped.

Answer: D

Correct answer: m – n, n

If the address is m bits and the page size is \( 2^{n} \), the low n bits are the offset within the page and the remaining m − n bits are the page number.

Answer: D

Correct answer: brings pages into memory before they are referenced

Prepaging brings pages into memory in anticipation of their being needed, rather than waiting for a reference to fault them in. Pure demand paging loads nothing until it is actually referenced. The motivation is the burst of faults a process suffers at start-up or on resuming after being swapped out, when none of its pages are resident. Loading its whole working set at once is cheaper than faulting it in page by page, because a single large transfer costs little more than a small one once the disk seek is paid for. Whether it pays depends on the accuracy of the guess. If a fraction \( \alpha \) of the prepaged pages are actually used, prepaging is worthwhile only when the cost saved on \( \alpha \) faults exceeds the cost of loading the \( 1-\alpha \) that were not needed. Where \( \alpha \) is near zero it is pure waste. Real systems use it selectively - on process resumption, where the working set is already known - and rely on demand paging otherwise.

Answer: A

Correct answer: rogue value

A rogue value is a value outside the range of the real data, placed at the end of a list to mark where it stops - a negative mark, or a name of three asterisks.

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.

Answer: B

Correct answer: Google

Google is a search engine; Internet Explorer and Firefox are browsers, which are how a search engine is reached, and Flash was a plug-in for animation.