Computer System Architecture Questions and Answers

Answer: B

The answer is a register.

Registers are a small set of very fast storage locations built into the processor itself. They hold the data, addresses and instructions the CPU is working on at that moment, for example the accumulator, the program counter and the instruction register. They are the fastest memory in the computer.

VLSI (very large scale integration) is a chip-making technology, not a storage place. A hard disk and a magnetic disk are secondary storage devices outside the processor, far larger and far slower than registers.

Answer: C

Correct answer: 4GHz

Clock rate is the reciprocal of the clock cycle time:

\[ \text{Clock rate}=\frac{1}{0.25\times10^{-9}\ \text{s}}=4\times10^{9}\ \text{Hz}=4\ \text{GHz} \]

A 0.5 ns cycle would give 2 GHz and a 0.125 ns cycle 8 GHz, which is what the other options correspond to.

Answer: A

Correct answer: Performance is increased when execution time is decreased

Performance is defined as the reciprocal of execution time, so the two move in opposite directions: halving the time doubles the performance. Any option in which both rise or both fall contradicts the reciprocal relation.

Answer: B

Correct answer: 1.5 times

Performance is the reciprocal of execution time, so the ratio of performances is the inverse ratio of times:

\[ \frac{\text{Performance}_A}{\text{Performance}_B}=\frac{\text{Time}_B}{\text{Time}_A}=\frac{15}{10}=1.5 \]

Computer A is therefore 1.5 times as fast as computer B.

Answer: A

Correct answer: Computer A

Compare the time per instruction, which is the clock cycle time multiplied by the cycles per instruction:

\[ A: 250\ \text{ps}\times 2.0=500\ \text{ps} \qquad B: 500\ \text{ps}\times 1.2=600\ \text{ps} \]

A takes 500 ps per instruction against B's 600, so A is the faster machine by a factor of 600/500 = 1.2. Note that B has the better CPI and A the better clock; neither figure decides the question by itself.

Answer: C

Correct answer: Interrupt

An interrupt is a signal raised by something outside the CPU - a device, a timer, an error condition - that causes the processor to break off what it is doing and attend to it. A halt stops the processor rather than diverting it, and a process is a program in execution.

Answer: A

Correct answer: Save it

Before servicing an interrupt the CPU saves its current state - the program counter and the registers - so that when the service routine finishes it can resume the interrupted program exactly where it left off. Discarding the state would make return impossible, and finishing the current program first would defeat the purpose of interrupting.

Answer: B

Correct answer: Performance_X > Performance_Y

The statement is simply restated in symbols: if X performs better than Y then its performance figure is the larger one. Since performance is 1/execution time, this is the same as saying X's execution time is the shorter of the two.

Answer: C

Correct answer: Synchronous

When the CPU and the interface share a common bus they are driven by the same clock, so the transfer needs no separate timing agreement between them - that is what makes it synchronous. Asynchronous transfer is used precisely when the two units have independent clocks and must signal each other with control lines.

Answer: C

Correct answer: Clustered System

A clustered system is the other way of putting several CPUs to work: two or more complete computers, each with its own memory, joined over a network and sharing storage so that they act as one. A minicomputer and a supercomputer are classes of machine, not ways of combining CPUs.