AS · Practice questions · Buses, ports and processor performance

Explain the effect.

Six original Cambridge-style questions on buses, performance factors and ports, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Earn the marks

Name it, then explain it.

Address bus: one way; n lines address 2ⁿ locations.
Data bus: two way; wider means more bits per transfer.
Performance: processor type, cores, bus width, clock speed, cache; always give the reason.
Ports: USB (serial, power, auto-detect), HDMI (digital video and audio), VGA (analogue video only).

Recall

[3 marks]

Describe the purpose of the address bus, the data bus and the control bus.

Address bus: carries the address of the location to be accessed from the processor to memory (one way). ✓
Data bus: carries data and instructions between the processor, memory and I/O (both ways). ✓
Control bus: carries control and timing signals, such as read, write and clock, between components. ✓

Calculate

[2 marks]

A computer has a 20-bit address bus. Each memory location holds one byte. Calculate the maximum amount of memory that can be addressed. Give your answer in MiB.

2²⁰ = 1 048 576 locations. ✓
1 048 576 bytes = 1 MiB. ✓

Explain

[4 marks]

Explain how increasing (a) the clock speed and (b) the size of the cache can improve the performance of a computer.

(a) The clock produces more pulses per second. ✓
(a) So more fetch-execute cycles, and so more instructions, are completed per second. ✓
(b) More frequently used instructions and data can be held in fast cache close to the processor. ✓
(b) So fewer accesses to slower main memory are needed. ✓

Explain

[2 marks]

A user replaces a dual-core processor with an eight-core processor, but one program runs no faster. Explain why.

The program may not be written to be split into tasks or threads that run in parallel. ✓
So it can only use one core at a time (and the other cores are idle). ✓

Apply

[3 marks]

For each device, identify the most suitable port and give a reason: (a) a smart TV used as a second screen for a film, (b) a USB flash drive, (c) an old projector with a 15-pin analogue input.

(a) HDMI: carries high-definition digital video and audio together. ✓
(b) USB: serial data, supplies power and is detected automatically. ✓
(c) VGA: it carries the analogue video signal the projector expects. ✓

Analysis

[2 marks]

A student writes: "Increasing the width of the address bus means more data is sent with each instruction." Explain why this is incorrect.

A wider address bus increases the number of memory locations that can be addressed. ✓
It is a wider data bus that allows more bits to be transferred at once. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.