Memory Management Questions and Answers

Answer: A

Correct answer: permit the logical address space of a process to be noncontiguous

If a process need not occupy one unbroken run of memory, it can be fitted into whatever holes exist, and external fragmentation ceases to matter. That is the idea behind paging and segmentation.

Answer: B

Correct answer: very high speed logic

The page table is consulted on every memory reference, so any delay in it is added to every instruction. That is why those registers are built with the fastest logic available.

Answer: C

Correct answer: string constant

"***" is a fixed sequence of characters written into the program, so it is a string constant. NAMEINR, which is compared against it, is the string variable.

Answer: B

Correct answer: CPU registers

Registers are the fastest and smallest storage, sitting inside the processor and accessible in a single clock cycle. A processor typically has only a few dozen. The hierarchy descends from there: registers, then cache in several levels, then main memory in DRAM, then secondary storage on solid-state or magnetic disk, then tertiary storage on tape or optical media. At each step capacity rises, cost per bit falls, and speed falls - often by an order of magnitude or more. The hierarchy works only because of locality of reference. Programs concentrate their accesses in small regions of address space, so keeping the active region near the top gives most references the speed of the fast level at the cost of the slow one. Without locality the whole arrangement would be pointless. Volatility changes at the boundary between main memory and secondary storage: everything above loses its contents when power is removed. Registers are managed by the compiler, cache by hardware, and main memory by the operating system.

Answer: A

Correct answer: first fit, best fit, worst fit

First fit stops at the first hole large enough, so it examines fewer holes than the other two, which must scan the whole list to find the smallest or the largest. In storage utilisation first fit and best fit are close, but in speed first fit wins.

Answer: A

Correct answer: if then statement

The statement has a condition after IF and one action after THEN, and no alternative branch, which is what makes it an if then rather than an if then else.

Answer: D

Correct answer: called by other programs

A subroutine exists to be called: control passes to it, it does its work, and control returns to the point after the call. That is what distinguishes it from a variable or a constant.

Answer: A

Correct answer: Digital subscriber line (DSL)

DSL uses the same copper pair as the telephone but transmits at frequencies above the voice band, so it is far faster than a dial-up modem and the line remains usable for calls. Cable and fibre are faster still but do not use the phone line.

Answer: B

Correct answer: semicolon

In Pascal the semicolon separates one statement from the next. The final END takes a full stop instead.

Answer: A

Correct answer: the page-fault rate

Effective access time is dominated by the page-fault rate, because the two costs involved differ by so many orders of magnitude. A memory access takes on the order of 100 nanoseconds; servicing a page fault takes several milliseconds, since it requires a disk transfer. The relation is \[ \text{EAT} = (1-p) \times t_{m} + p \times t_{f}, \] where \( p \) is the fault rate. With a memory time of 100 ns and a fault time of 8 ms, a fault rate of just one in a thousand gives an effective time of about 8 microseconds - eighty times slower than memory alone. That arithmetic is why the fault rate must be kept extraordinarily low, of the order of one in a million, for demand paging to be worthwhile, and why locality of reference is essential to the whole scheme. It is also why thrashing is catastrophic rather than merely slow, and why replacement algorithms are studied so closely.