GCSE Computer Science Key Words

Key words for use at GCSE level and above computer science.


ALU stands for Arithmetic Logic Unit. The ALU is a vital part of the central processing unit.

The job of the ALU is to carry out calculations, adding, subtracting, multiplying and dividing. The ALU also carries out comparisons which answers questions such as, 'is the number 'A' greater, less than or equal to the number 'B'.

Assembly Language

Assembly language consists of a set of mnemonics that can be used to program a CPU. There are a number of mnemonics that together make up the complete instruction set of the CPU. They can be grouped according to the kind of processing they cover.


Binary logic is much the same as general logic, except it works on 1's and 0's. Given one or more binary inputs a logical output will result.

It is common practice to consider binary 1 to be 'True' and binary '0' to be 'False'.

There are only a few simple logic operations but they do lead to incredibly complex devices such as a CPU.


A bit is the smallest amount of information that a computer can store. It can be set to 0 (false) or 1 (true).


Boolean usually refers to logic.The most common statement in logic are AND, OR, NOT. A boolean search combines key words together using logic statements to link them together.


A Byte is 8 Bits. A string of eight numbers, either 1 (true) or 0 (false). e.g: 01100101

Cache Memory

It is used to speed up the way applications work. For example, Internet Explorer uses a cache to store all the web pages that you have visited recently including all of the pictures, sounds, videos and so on. So, next time you visit that page it will check to see if it is available in the cache. If it is, then it will instantly load the page from the cache rather than having to download it again from the Internet. This makes it much faster when you browse the Internet as it doesn't have to fetch every single file every time.

Clock Speed

Inside the Central Processing Unit (CPU) there is a 'clock' - the CPU clock 'ticks' by switching from high to low and then back again in a very precise time:

Every time the clock pulses an instruction is carried out: Clock speed is measured in pulses per second. Since "1 pulse per second" is also known as 1 Hertz computer speed is often specified in 'Gigahertz' (GHz). The CPU clock speed on personal computers runs from less than 1GHz to about 3.8GHz

Command Line

A command line interface or CLI is a type of User Interface which enables you to interact with a computer by typing in a series of text commands. MS-DOS is an example of a command line interface. CLIs require you to know the exact, specific commands in order to make something happen. They are unforgiving; any mistakes will result in an error message.


A CPU (Central Processing Unit) is the core of every Personal Computer. Without it, no PC can function. The CPU is a microchip that is installed on a motherboard and acts as the computer’s brain - performing calculations and coordinating the hardware components.


Effectively a dual core is two computers built into one device. The downside of multi-core chips is that whatever software is running has to make full use of the cores - and right now, there isn't much software around that does that.

Fetch-Execute Cycle

A standard process describes the steps needed for processing to take place. It is called the Fetch - Decode - Execute cycle or sometimes simply called the Fetch-Execute Cycle.


The first step the CPU carries out is to fetch some data and instructions (program) from main memory then store them in its own internal temporary memory areas. These memory areas are called 'registers'.


The next step is for the CPU to make sense of the instruction it has just fetched.

This process is called 'decode'. The CPU is designed to understand a specific set of commands. These are called the 'instruction set' of the CPU. Each make of CPU has a different instruction set. The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step.


This is the part of the cycle when data processing actually takes place. The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register. Once the execute stage is complete, the CPU sets itself up to begin another cycle once more.

Grafical User Interface

A GUI is a graphics-based user interface that incorporates windows (W), icons (I), menus (M)and pointers (P). You often hear a gui referred to as a WIMP interface.


Integer refers to a number data type which will only store whole numbers. It will not store decimal numbers or fractions.


The Motherboard is the main printed circuit board in the computer. It has many sockets into which the CPU, memory (RAM and ROM), cards (sound, graphics, network interface etc) and peripheral devices are connected. The Motherboard is essential as a means of connecting all of the computer's parts together.


Metadata is a fancy term for 'data about data'. In other words metadata describes the structure of the data file. Metadata is usually located right at the start of the file. When a media player opens the file it looks for the metadata which describes the format of the file e.g. the file type, how many horizontal pixels are there, how many vertical pixels and the colour depth of the file. Armed with this infomation the media player reads the file and correctly displays the image.

Operating System

The operating system is part of the system software. All computers have an operating system, they cannot function without one. The operating system is a program that allows applications software to communicate with the hardware.


This is the same as if you are not allowed to have a decimal number greater than 255 because of a computer limit but the software is trying to add 249 + 21. This is 270 but since the system cannot deal with numbers larger than 255 an overflow occurs.

Overflow is such a problem in a CPU that a special 'flag' inside the CPU is set when it happens. The software carrying out the maths should read this flag to see if an overflow has occured. If it has then it has to deal with the problem.


Pixel is short for Picture Element.A pixel is the smallest individual dot that can be displayed on a computer monitor. Each pixel is given a colour and brightness value.An image is made up of many pixels. The quality of the image depends on the number of pixels per inch that make up the image.


Pseudo-code does not follow any particular computer language. It lays out the algorithm as a series of statements written in English (or any local language). Some statements will test for some condition and branch to different parts of the algorithm.


Random access memory (or simply RAM) is the memory in a computer that is used to store computer programs while they are running any information the programs need to do their job. Usually, the RAM is emptied every time the computer is turned off. It is known as 'volatile memory'.


The data held on ROM can be read but not changed. This is done during manufacturing. ROM is non volatile, meaning that the data stored on it will not be lost when the computer is switched off.


A 1 byte scheme can only represent 256 symbols. This is fine for many individual languages which is why ASCII is so popular.

Virtual Memory

This is a method of making a computer appear to have more primary memory than it has really has physically. For instance a PC might have 2Gb of RAM as primary memory, but virtual memory of 4Gb.