Computer Science - AQA A Level

Control Unit

Coordinates activities in the CPU, memory and peripherals

Clock

A continuous sequence of clock pulses to step the control unit through its operations

Arithmetic Logic Unit (ALU)

The part of the central processing unit that performs arithmetic computations and logical operations

Registers

Very fast memory stored on or near the processor

Program Counter (PC)

Holds the address of the next instruction to be executed

Current Instruction Register (CIR)

Stores the current instruction (in binary)

Memory Address Register (MAR)

Holds the address of a memory location from which data will be read or data will be written to

Memory Buffer Register (MBR)

Temporarily stores the data read from or written to memory

Status Register (SR)

Contains many bits that are set (1) or cleared (0) depending on the result of an instruction

Clock Speed

The frequency at which the system clock ticks

Bus Width

The number of parallel signal lines or wires allocated to a bus

Word Length

The number of digits in a binary word

Cache Memory

A small amount of very fast memory

Increasing Clock Speed

Reduces the time taken to execute instructions

Increasing Word Length

Registers can store more bits and the processor can process more bits in one go

Increasing Data Bus Width

Enables more bits to be transferred between main memory and the processor at one time

Increasing Address Bus Width

Enables the processor to access a larger number of main memory locations meaning it will not need to make as much use of secondary storage

Increase Amount Of Cache Memory

Increases speed and response time as more can be stored in cache memory and main memory needs to be accessed less often

Multicore Processors

Increases throughput but greatly increases complexity and heat production

Increase The Number Of General Purpose Registers

More intermediate results/variables can be kept in processor registers instead of main memory

Radio Frequency Identification (RFID)

A form of contactless identification which uses radio waves to transmit and receive data

Digital Camera

A type of camera that is able to save images and videos digitally, normally to an SD card or other form of EMMC storage

Barcode Reader

A device used to read a small pattern of black and white parallel lines and numbers, also known as a barcode

Laser Printer

A type of printer that uses electrical charges to create an image on paper

Hard Drives

A non-volatile, magnetic form of internal or external storage, that permanently stores and retrieves data on a computer

Solid State Drive (SSD)

A non-volatile type of mass storage similar to a hard drive that supports reading and writing data, and stores data even without power

Optical Disk

A form of secondary storage that uses light emitted by a laser to read and write onto a disk.

Encapsulation

Combining properties and methods together into an object

Normalisation

The process of decomposing a flat-file into the best possible design for a relational database

Encryption

Using an algorithm to change data so that it can only be read by the intended recipient

Cipher

The algorithm which is used to encrypt or decrypt

Plaintext

The unencrypted message

Ciphertext

The encrypted message

Symmetric Encryption

The same key is used for both the encryption and decryption algorithm, and for the whole message

Asymmetric Encryption

Different keys are used to encrypt and decrypt

Machine Code

Code written in binary which can be read by the computer

Instruction Set

All of the different possible instructions the computer can carry out

Operation Code

Part of the binary code which tells us what operation is being carried out

Operand

Part of the binary code which tells us what to carry out the instruction on

Assembly Form

Machine code that can easily be read by humans

Assembler

Converts assembly language into machine code that the computer can run

Source Code

The code being translated

Interpreter

Translates code from a high level language to computer hardware. Code is run after each line is translated, so it doesn't create 1 piece of machine code

Compiler

Translates from high level language to machine code, to computer hardware. A one off translation from source code to machine code

Object Code

The code we translate to - usually a big chunk of machine code

Hypertext Transfer Protocol (HTTP)

Port 80 - used for viewing web pages

Hypertext Transfer Protocol Secure (HTTPS)

Port 443 - used for accessing secure web pages

Secure Shell (SSH)

Port 22 - used for remotely accessing and managing a computer. Uses public key encryption

Post Office Protocol (POP)

Port 110 - used for downloading personal emails from a web sever

Simple Mail Transfer Protocol (SMTP)

Port 25 - used for sending email messages

File Transfer Protocol (FTP)

Port 21 - used for uploading files to a web server

Natural Numbers

Positive integers

Integers

Whole numbers, positive or negative

Rational Number

Any number that can be expressed as a ratio of two numbers e.g. 2/3

Irrational Numbers

A real number that cannot be expressed as a ratio of two numbers e.g. pi or e

Real Number

Any positive or negative number, including integers, rational and irrational numbers

Ordinal Number

Used to describe the order type of a well-ordered set, e.g. y=ax+b

Cardinal Numbers

A generalisation of the natural numbers used to measure the size of sets

Abstraction

Simplified models to describe complicated systems and processes

Stored Program Concept

A program must be resident in main memory to be executed

Harvard Architecture

Instruction memory and data memory are stored separately

Von Neumann Architecture

Instruction memory and data memory are stored together

Fetch

The contents of the PC are copied into the MAR. The instruction held at that address in main memory lands into the MBR. The PC is incremented to hold the address of the next instruction to be executed. The contents of the MBR are copied to the CIR.

Decode

The instruction held in the CIR is decoded

Execute

The ALU is used for mathematical and logic instructions. For load and store instructions, the MAR and MBR are used to access data from main memory. The general purpose registers and SR are updated.

Call Stack

Stores all information about 'active' procedures / subroutine / function calls. Each time a subroutine is called, a new stack frame is created. Each time a subroutine is terminated, the associates stack frame is removed.

Static Memory

Allocated to values of fixed size which we can allocate before running the code

Dynamic Memory

Allocated to values of unknown size. We do not know how much space we will need to store them. These are called objects.

Transmission Control Protocol/Internet Protocol (TCP/IP)

A communication protocol for computers on the internet. It defines how computers should be connected to the internet and how data should be transmitted between them

Application Layer

Gives applications access to the network

Transport Layer

Provides reliable transmission of data. Puts data into packets and gives them a header containing the port numbers of source and destination and the sequence number. Handles errors (acknowledgements, timeouts and re-transmitting when necessary)

Network Layer

Where the routing occurs. It adds the source and destination IP addresses. Fast but potentially unreliable

Link Layer

Physical interface with the cable. This includes the network card and a device driver. Adds the MAC address of source and destination to the header.

Methods

Subroutines within an object that are designed to carry out a process using the data inside an object.

Properties

The defining features of an object

Class

Defines the properties and methods of an object

Object

A specific instance of a class

Inheritance

The properties an methods of a class can be inherited by a subclass

Instantiation

The process of creating an object from a class

Polymorphism

Where more than one method in a class/subclass has the same name, but take different parameters and does different things

Composition Aggregation

Creating an object that is composed other objects, and will cease to exist if the containing object is destroyed

Association Aggregation

Creating an object that contains other objects, which can continue to exist even if the containing object is destroyed

MIDI

Musical Instrument Digital Interface - a way to connect devices that make and control sound

MIDI Event Messages

Data that specify the instructions for music, including a note's notation, pitch, volume, vibrato and tempo

Advantages of using MIDI

Small file size, ease of modification and manipulation and a wide choice of electronic instruments and synthesizer or digitally-sampled sounds

Serial Data Transmission

In serial data transmission, single bits are sent one after another along a single wire

Parallel Data Transmission

In parallel data transmission, bits are sent down several wires simultaneously. The connecting cable consists of many wires and is called a parallel bus.

Advantages of serial over parallel

Parallel has a limited data rate and distance at which it can be reliably operated compared with serial. The limited data rate and distance of parallel data transmission are caused by skew (the phenomenon where the bits travel at slightly different speeds down each wire in a parallel bus) and crosstalk (induced signals in adjacent wires of a parallel bus caused when a signal on one or more wires varies rapidly)

Bit Rate

the number of bits transmitted per second (voltage)

Baud Rate

The maximum rate at which signals on a wire or line may change. 1 baud = 1 signal change per second.

Relationship between Baud Rate and Bit Rate

bit rate = baud rate x number of bits per signal

Bandwidth

The bandwidth of a transmission medium (e.g. copper wire) is the range of signal frequencies that it may transmit from one end of the communication link to the other without significant reduction ion strength of the signal (Hz)

Latency

The time delay that can occur between the moment something is initiated and the moment its first effect begins.

Synchronous Data Transmission

Synchronous serial data transmission is a form of serial communication in which the communicating endpoints' interfaces are continuously synchronized by a common clock.

Asynchronous Data Transmission

Asynchronous means that data is transferred without support from an external clock signal. No clock wire is required.

Start Bits

The start bit signals the arrival of data at the receiver. As the arrival of data cannot be predicted by the receiver, the transmission is called asynchronous.

Stop Bits

The voltage level chosen for the stop

bit is the level for binary digit 1. Two stop bits are used if the receiver needs more time to deal

with the received bits.

Parity Bit

A method of checking binary codes by counting the number of 0s and 1s in the code.

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