Computer System Architecture Questions and Answers

Answer: B

Registers store data, instructions and intermediate results.

Registers are tiny, very fast storage locations inside the CPU. While the processor works, they hold the instruction being executed, the data it is acting on, memory addresses and the intermediate results of calculations, so that the CPU does not have to go to slower main memory for every step.

Long-term data is kept in secondary storage such as a hard disk. Arithmetic operations are carried out by the ALU, which uses registers but is not itself a register. Directing data flow is the job of the control unit and the buses.

Answer: B

Data is cached to reduce average access time.

Cache memory is a small, very fast memory placed between the CPU and main memory. It keeps copies of the data and instructions used most often or most recently, so that the processor can get them quickly instead of waiting for the slower RAM. This lowers the average time taken to access memory.

A cache does not save storage space; it adds another copy. It does not keep irrelevant data, since the point is to keep data likely to be needed again. And it is temporary, volatile memory, not a permanent archive.

Answer: B

The bootstrap program is stored in ROM.

The bootstrap loader is the small program that runs when a computer is switched on: it tests the hardware and loads the operating system into memory. It must be available the moment power comes on, so it is kept in read-only memory (or firmware such as BIOS or UEFI chips), which is non-volatile and keeps its contents without power.

RAM is volatile and is empty at start-up. The hard disk holds the operating system that the bootstrap program loads, but the loader itself is not run from there. A pen drive is removable storage.

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.