Memory Management Questions and Answers

Answer: B

Correct answer: Modem

A telephone line carries an analogue signal, so a modem is needed to convert the computer's digital data for transmission and back again on arrival.

Answer: C

Correct answer: It should be deleted without opening it

An unexpected message from an unknown sender may carry malware in its attachment or a link to a fraudulent site, and replying at all confirms that the address is live. Deleting it unopened is the only response that gives the sender nothing.

Answer: D

Correct answer: the page that has not been used for the longest time

LRU evicts the page whose most recent use lies furthest in the past, on the reasoning that a page unused for a long time is unlikely to be needed soon - the principle of temporal locality. It approximates the theoretical optimal algorithm, the third option, which evicts the page that will next be needed furthest in the future. That is unimplementable, since it requires knowledge of the future, but it is used as a benchmark against which real algorithms are measured. FIFO, the second option, evicts the oldest-loaded page and is simple but poor, and it suffers Belady’s anomaly: adding more frames can increase the number of page faults, which is deeply counter-intuitive. LRU belongs to the class of stack algorithms and cannot exhibit it. True LRU is expensive to implement, needing a timestamp or a stack updated on every reference, so real systems approximate it with reference bits - the clock or second-chance algorithm being the usual method.

Answer: C

Correct answer: A multi-homed firewall

A multi-homed firewall has an interface on each network it joins, so it both separates the subnets and applies rules to everything passing between them and the outside. A router or VLAN separates without filtering, and a modem does neither.

Answer: A

Correct answer: replacement algorithms that approximate LRU

The reference bit is set by hardware whenever a page is read or written, and cleared by the operating system. It is used by replacement algorithms that approximate LRU. The need arises because true LRU is too expensive. Maintaining an exact ordering of pages by last use requires a timestamp or a stack updated on every memory reference, which no practical hardware provides. The reference bit is set for free as a side effect of the access, and one bit is enough to distinguish pages used since the last sweep from pages not used at all. The second-chance or clock algorithm uses it directly: a page at the head of the queue with its bit set is spared and the bit cleared, so only a page unused since the last sweep is evicted. The additional-reference-bits refinement keeps a byte per page, shifting the reference bit in at intervals, which gives a coarse history rather than a single bit and approximates LRU more closely at modest cost.

Answer: B

Correct answer: allocated memory is slightly larger than the requested memory, leaving unused space inside a block

Internal fragmentation is space wasted inside an allocated block, because memory is handed out in fixed units and the request rarely fills the last one exactly. A process needing 5 KB with a 4 KB page size receives two pages and wastes 3 KB. External fragmentation, the second option, is the opposite: enough free memory exists in total but it is scattered in pieces too small to satisfy a request. The distinction determines the remedy. Paging eliminates external fragmentation entirely, since any free frame will do for any page, but introduces internal fragmentation of at most one page per process - on average half a page. Contiguous allocation and segmentation avoid internal fragmentation but suffer external. Compaction cures external fragmentation by shuffling processes together to make one large hole, at considerable cost in copying, and only works if relocation is dynamic.

Answer: A

Correct answer: Hit

Each matching page a search engine returns is a hit. A link is what a page contains and a blog is a kind of site.

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: C

Correct answer: Both 1 and 2

A firewall does both: it blocks traffic that should not reach the network at all, and it applies rules deciding which of the permitted traffic may reach which internal resources.

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.