What does the Computer Systems area cover in SQA Higher Computing Science?

It is part of the mandatory section of the question paper, examined alongside Software Design and Development. It covers low-level data representation: storing positive and negative integers in two's complement and real numbers in floating-point with a mantissa and exponent; representing characters with ASCII, extended ASCII and Unicode; the structure of a computer (the processor's ALU, control unit and registers, the data and address buses, and memory); the difference between an interpreter and a compiler; and the environmental impact of computer systems including intelligent systems.

How are negative numbers stored using two's complement?

To represent a negative integer in two's complement, you write the positive value in binary, invert every bit (0 becomes 1 and 1 becomes 0), then add 1. The most significant bit then acts as a sign bit: 0 for positive, 1 for negative. Two's complement is used because it lets the processor subtract by adding the negative, so one adder circuit handles both addition and subtraction, and there is only one representation of zero.

What is the difference between a compiler and an interpreter?

A compiler translates the whole high-level program into machine code in one go, producing a separate executable file that is then run; the result runs quickly and can be shared without the source code. An interpreter translates and executes the program one instruction at a time, every time it runs, without producing a separate executable; this makes debugging easier and the code more portable, but execution is slower because each line is translated each time it runs.

What do the data bus and address bus do?

The address bus carries the location (address) of the memory cell the processor wants to access; it is one-directional, from the processor to memory, and its width sets how many locations can be addressed. The data bus carries the actual value to or from that location and is bidirectional, because it carries data to memory on a write and from memory on a read. For a read, the address bus selects the location and the data bus brings the value back to a register.

How does computing affect the environment, and how can the impact be reduced?

Computer systems consume large amounts of electricity, much of it from fossil fuels, producing carbon emissions; data centres add demand for cooling, and manufacturing and disposal create resource use and e-waste. The impact can be reduced with energy-efficient hardware, power-saving settings, switching equipment off when idle, renewable energy, efficient cooling, virtualising servers and recycling old equipment. Intelligent systems such as large machine-learning models are especially energy-intensive, though they can also help cut impact elsewhere.

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SQA Higher Computing Science Area Computer Systems: a complete overview of number and character representation, computer structure, translators and environmental impact

A deep-dive SQA Higher Computing Science guide to the Computer Systems area. Covers number representation (two's complement and floating-point), character and instruction representation (ASCII and Unicode), computer structure (processor, buses, memory, interpreters and compilers) and the environmental impact of computing.

Generated by Claude Opus 4.816 min readHigherUpdated 2026-06-16

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Computer Systems actually demands
Number representation
Character and instruction representation
Computer structure
Environmental impact
How Computer Systems is examined
Check your knowledge

What Computer Systems actually demands

Computer Systems sits in the mandatory section of the Higher question paper, examined alongside Software Design and Development. It moves from the lowest level of representation (how binary stores numbers, text and instructions) up to the structure of the machine and out to the environmental consequences of computing. The examiners reward correct binary working, precise named components, and clear comparisons (two's complement versus floating-point, compiler versus interpreter, the two buses).

This guide walks through the key areas of Computer Systems, then sets out the patterns the SQA repeats. Each key area has a matching dot-point page with practice questions; this overview ties them together.

Number representation

The area opens with number representation. Positive integers are plain binary; negative integers use two's complement (invert the bits and add 1, with the most significant bit as a sign bit), chosen because it lets the processor subtract by adding. Real numbers use floating-point: a mantissa for the significant digits and an exponent for the scale, with a fixed-bit trade-off between precision (mantissa) and range (exponent).

Character and instruction representation

Characters are stored using a character set that maps each symbol to a numeric code: ASCII (7 bits, 128 codes), extended ASCII (8 bits, 256 codes) and Unicode (many more bits, over a million codes, covering every writing system at the cost of more storage). The key principle is that all data and all instructions are binary, and a pattern's meaning depends on how it is used.

Computer structure

The structure of a computer centres on the processor: the ALU (calculations and logic), the control unit (fetch-decode-execute) and registers (fast internal stores). The processor reaches memory over two buses: the address bus (one-way, carries the location) and the data bus (two-way, carries the value). Translators turn high-level code into machine code: a compiler (whole program at once, separate executable) or an interpreter (line by line each run).

Environmental impact

The environmental impact of computing comes mainly from energy consumption and the emissions behind it, plus cooling, manufacturing and e-waste. It is reduced by efficient hardware, power management, renewable energy, efficient cooling, virtualisation and recycling. Intelligent systems (large machine-learning models) are especially energy-intensive, though they can also help reduce impact elsewhere.

How Computer Systems is examined

A typical SQA profile for Computer Systems:

Binary working. Converting small values to two's complement, and reasoning about floating-point precision and range.
Description. Naming processor parts, bus roles, character sets, and translator behaviour.
Explanation. Why two's complement is used, the precision/range trade-off, compiler versus interpreter advantages, and environmental measures.

Check your knowledge

A mix of recall, binary and explanation questions covering Computer Systems. Attempt them, then check against the solutions.

State the two steps to negate a number in two's complement. (1 mark)
State which part of a floating-point number sets its precision. (1 mark)
State how many codes standard 7-bit ASCII provides. (1 mark)
Name the processor component that performs calculations and logic. (1 mark)
State which bus carries the memory location and its direction. (1 mark)
State which translator produces a separate executable file. (1 mark)

Sources & how we know this

SQA Higher Computing Science Course Specification (C816 76) — SQA (2023)