Memory Management Questions and Answers

Answer: A

Correct answer: local variable

A variable declared inside a subroutine and visible only there is local. Confining it prevents the routine from disturbing anything outside it.

Answer: B

Correct answer: each physical frame of memory

An inverted page table has one entry per physical frame rather than per virtual page. Each entry records which process and which virtual page currently occupy that frame. The motivation is size. An ordinary page table exists per process and covers the whole virtual address space, so with many processes and large address spaces the tables consume a great deal of memory. An inverted table is a single system-wide structure whose size is fixed by physical memory, which is far smaller. The cost is the search. Translation now requires finding the entry containing a given process and page number, which is a search rather than an index - potentially through the whole table. Real implementations use a hash table to bound it, and the TLB to avoid it in most cases. A second drawback is that shared memory becomes awkward, since one frame can have only one entry, and shared pages belong to several processes at once. The scheme has been used on architectures such as PowerPC and UltraSPARC.

Answer: C

Correct answer: pointers to the inner page tables

In a two-level scheme the outer table holds pointers to inner page tables, and the inner tables hold the frame numbers. The logical address splits into three parts: an index into the outer table, an index into the inner one, and an offset. The reason for the arrangement is size. A 32-bit address space with 4 KB pages needs a million page-table entries per process, which at four bytes each is 4 MB of contiguous memory - unacceptable for every process. Splitting the table means only the outer table and the inner tables actually in use need exist; the rest are simply absent, and most processes use a small part of their address space. The cost is another memory access per translation, which is why the TLB matters so much. 64-bit architectures need three or four levels for the same reason, and the alternative approaches are the hashed page table and the inverted page table, which keeps one entry per physical frame rather than per virtual page.

Answer: B

Correct answer: larger than the hole itself

A hole must be recorded in the free list, and if the hole is very small the record costs more memory than the hole would ever supply. Such holes are therefore left attached to the allocated block, which converts external fragmentation into internal.

Answer: A

Correct answer: identifier

NUM is a name the programmer chose, which is what an identifier is. Its being chosen rather than fixed by the language is what distinguishes it from a keyword.

Answer: A

Correct answer: A smaller page table

A larger page size means fewer pages for a given address space, so the page table is smaller - which saves memory and improves TLB coverage, since each entry maps more of the address space. Larger pages also make disk transfers more efficient, because reading one large block costs little more than reading a small one once the seek is paid for. The costs run the other way. Internal fragmentation rises, since the waste is on average half a page per process, so the second option is precisely backwards. And more data is transferred that may never be used, since a fault brings in the whole page. Protection also becomes coarser: permissions apply per page, so a larger page is a blunter instrument. The trade-off has settled on 4 KB for most systems, with support for huge pages of 2 MB or 1 GB where a workload - a large database, a virtual machine host - benefits from the reduced TLB pressure. Page size is always a power of two, so that address division needs no arithmetic.

Answer: B

Correct answer: standard function

Functions supplied with the language and available to every program - square root, absolute value, string length - are the standard functions.

Answer: A

Correct answer: procedure

Stepwise refinement breaks a task into smaller tasks, and each is written as a procedure. That is what allows the top level of the program to read as a summary of what it does.

Answer: B

Correct answer: Designing

E-mail, newsgroups and chat are all services carried over the internet. Designing is something done with software on a machine, whether or not it is connected.

Answer: A

Correct answer: hardware support such as a relocation register

Binding logical addresses to physical ones at execution time requires hardware support, classically a relocation or base register whose contents are added to every address the process generates. The memory management unit performs the addition on each reference. Its advantage is that a process can be moved in memory during execution, since only the register need change. That is what makes swapping and compaction possible, and it is the basis of every modern system - paging being the general form of the same idea, with a page table in place of a single register. The alternatives bind earlier and are less flexible. Compile-time binding produces absolute code that must be loaded at one fixed address; changing it requires recompilation. Load-time binding produces relocatable code fixed when the program is loaded, so it can start anywhere but cannot move afterwards. A limit register accompanies the base register to enforce protection, trapping any address beyond the process’s allocation.