AQA A-Level Computer Science

A mapping from one set of values to another. The result of this mapping is unique to a given set of inputs, and a value must always be returned

Using a procedure to carry out a sequence of steps for achieving some task

Where data is not directly accessible and can only be accessed through defined procedures/functions

Breaking down a complex problem into a number of sub-problems, each of which performs an identifiable task

Combining procedures to form compound procedures (e.g. BuildHouse, BuildStreet). It can also mean combining objects to form compound data, for example records or a data structure such as a queue, tree or list.

Building and putting into action models to solve problems

Abstract model of computation used to design computer programs and sequential logic circuits. It can be in one of a finite number of states and changes from one state to the next state when triggered by some condition or input (say, a signal from a timer).

Finite state machine which has no output

In a finite state machine, when you are in a particular state, and the next state is uniquely determined by the input

Any whole number

A whole number used in counting

Any value that can be expressed as a ratio, or fraction, including all integer values

A value that cannot be expressed as a fraction and has an endless series of non-repeating digits

Describe the numerical positional of objects

Additional bit that is used to check that the other bits transmitted are likely to be correct

The total number of 1s in each bye, including the parity bit, equals an odd number

The total number of 1s in each bye, including the parity bit, equals an even number

System that requires each bit to be sent three times. If a bit value is flipped erroneously during transmission over a noisy line, the recipient computer would use the majority rule and assume that the two bits that have not changed were therefore correct

Mathematical algorithm that is applied to a ‘unit’ or packet of data. The data in the block is used to create a checksum value which is transmitted with the block. The algorithm is applied to the block after transmission and if the two checksums match, the transmission is deemed to have been successful.

Similar to a checksum. An additional digit at the end of a string of other numbers designed to check for mistakes in input or transmission.

Can represent only positive numbers

Can represent both positive and negative numbers

One representation of a signed binary number

A way to represent fractions in binary. A binary point is used to separate the whole place values from the fractional part.

Contain pixels that make up an image

A smallest identifiable area of an image, attributed a binary value which represents a single colour

Width in pixels * height in pixels

Pixels per inch

Indicates the density of pixels within an image

The number of bits per pixel, which determines the number of colours that a pixel can represent

Data about data

• Width/height in pixels

• Colour depth

• GPS coordinates where the image was taken

• Camera or device used to take the image

• Exposure time

• Focal length

Images made up of geometric shapes or objects. The file contains only the necessary details about each shape in order to redraw the object when the file loads.

Properties of each object include:

• Position

• Radius

• Fill colour

• Line colour

• Line weight

The frequency with which you record the amplitude of the sound

Number of samples per second * Number of bits per sample * Length of the sample in seconds

A continuous signal which represents physical measurements

Discrete time signals generated by digital modulation

Uses a continuous range of values to represent information

Uses discrete or discontinuous values to represent information

1. Microphone converts sound energy into electrical energy

2. ADC (analogue to digital converter) samples the analogue data at a given frequency, measuring the amplitude of the wave at each point and converting it into a binary value according to the resolution or audio bit depth being used for each sample

1. Binary values for each sample point are translated back into analogue signals or voltage levels

2. These are sent to an amplifier connected to a speaker, producing sound

The speed of oscillation or vibration of a wave, measured in Hertz (Hz)

In order to produce an accurate recording, the sampling rate must be at least double that of the highest frequency in the original signal

Musical Instrument Digital Interface

A technical standard that describes a protocol, digital interface, and connectors which can be used to allow computers and electronic musical instruments to connect and communicate

• Carries event messages that:

  • Specify:

    • Pitch and duration of a note

    • Timbre

    • Vibrato

    • Volume changes

  • Synchronise tempo between multiple devices

List of instructions that tell it to synthesise a sound based on pre-recorded digital samples and synthesised samples of sounds

Unnecessary information is removed from the original file

All the information required to replicate the original file exactly is retained

Run Length Encoding

A method of lossless compression whereby repetitive and consecutive data is replaced with counts of how many are in a row

A method of

The transformation of data from one form to another to prevent an unauthorised third party from being able to understand it.

The original data or message before encryption

The encrypted data

The secret information to lock or unlock the message

A type of substitution cipher that works by shifting the letters of the alphabet along by a given number of characters, the number of characters being the key

A type of one-time pad cipher that offers perfect security. A Boolean XOR operation is carried out between the binary representation of each character of the plaintext and the corresponding character of the one-time pad

To provide perfect security, the encryption key must be:

• equal to or longer in characters than the plaintext

• truly random

• used only once.

The sender and recipient must meet in person to securely share the key and subsequently destroy it after encryption and decryption.

Not susceptible to cryptanalysis due to its truly random distribution of characters.

The electrical or electro-mechanical parts of a computer and its input, output, and storage devices

All the programs that are written to make computers function

The software needed to run the computer’s hardware and application programs

A set of programs that lies between applications software and the computer hardware.

It provides an Application Programming Interface (API) for hardware and software to communicate.

Its functions include:

• Resource management

  • Memory management

  • Processor scheduling

  • Backing store management

  • Management of all input and output

• Provision of a user interface

Managing all the computer hardware including the CPU, memory, disk drives, keyboard, monitor, printer, and other peripheral devices

Enables users to perform tasks like:

• Running application software

• Changing settings on the computer

• Downloading and installing new software

The allocation and deallocation of memory space for processes, programs, and data.

Virtual memory may be used.

Allocating processor time to each process in the most efficient way possible, allowing for multi-tasking

• Maximise throughput

• Be fair to all users on a multi-user system

• Provide acceptable response time to all users

• Ensure hardware resources are kept as busy as possible

A secondary storage device that holds data and programs that are not currently in use by the main memory

• Allocating space on the backing store

• Transferring data and programs between the main memory and the backing store

• Keeping track of the location and status of each file on the backing store

• Keeping track of free areas of storage so new files or applications can be saved

• Allowing users to move files and folders, delete files, and protect others from unauthorised access

Using device drivers to control the sending and receiving of data to and from peripheral input and output devices. The operating system also ensures peripherals are allocated to processes without causing conflicts.

A signal from a peripheral or software program that causes the operating system to stop processing its current list of instructions

When an error occurs and a process interrupted, the operating system must detect the interrupt signal and display an appropriate error message (if appropriate) before deciding what to do next. Interrupts are what allows multi-tasking to take place.

System software designed to optimise the performance of the computer or perform tasks like:

• backing up files

• restoring corrupted files from backup

• compressing or decompressing data

• encrypting data before transmission

Program that will reorganise a hard disk so that files which have been split up into blocks and stored all over the disk will be recombined into a single series of sequential blocks

Checks your hard drive, incoming emails, and internet downloads for viruses, and removes them

Ready-compiled programs that can be run when needed

Groups of library programs. Most compiled languages have their own libraries of pre-written functions that can be invoked in a defined manner from within the user’s program.

A translator than translates program code written by a programmer into machine code which can be run by the computer

A translator that translates a high-level language into machine code. It considers the entire code and converts it at once, instead of translating code line-by-line.

A translator that contains subroutines to carry out each high-level instruction. It looks at each line of the source program, analyses it, and, if it contains no syntax errors, calls the appropriate subroutine within its own program code to execute the command.

An intermediate representation of a program’s source code combining compiling and interpreting. It is low-level code that is designed for a software interpreter, compiled to run on a virtual machine instead of a CPU.

Platform independence

An example of a bytecode interpreter that understands Java bytecode and converts it into the machine code for the particular computer it runs on

• The object code can be saved on disk and run whenever required without the need to recompile

• The object code executes faster than interpreted code

• The object code produced by a compiler can be distributed or executed without having to have the compiler present

• The object code is more secure, as it cannot be read without ‘reverse engineering’

• When a program is to be run regularly or frequently, with only occasional changes

• When the object code produced by the compiler is going to be distributed or sold to users outside the company that produced the software (as source code is not present)

• It is useful for program development as there is no need for lengthy recompilation every time an error is discovered

• It is easier to partially test and debug programs

• During program development

• In a student environment when students are learning code, as they can test parts of a program before coding it all

Translates an assembly language program (source code) into the 0s and 1s of the corresponding machine code instruction (object code) line-by-line.

Programs that perform specific user-oriented tasks

Application software that can be used for many different purposes, e.g.:

• word processors (Word)

• spreadsheet packages (Excel)

• graphics software (Photoshop)

Application software that performs a single specific task or a set of tasks, e.g.

• payroll and accounts packages

• hotel booking systems

• fingerprint scanning systems

• browser software

Can be bought ‘off-the-shelf’, ready to use, or can be specially written by a team of programmers for a particular organisation.

Inverts the input

The output is only 1 when both A and B are 1

The output is only 1 when either A, B, or both A and B are 1

Exclusive or; the output will be true if one or other input is true, but not both

Inverts the output of the AND gate

Only produces an output of true when both inputs are false

NOT(A + B) = NOT(A) • NOT(B)