# A-level Computing 2009/AQA/Print version/Unit 2/Definitions

* Boolean variable: has two discrete values: true or false
* Truth Table: a table that shows the results of applying a logical operation
* Boolean equation: an equation that expresses a boolean output in terms of boolean inputs and the functions applied
* Logic gate: an electronic circuit that performs a boolean function
* Hardware: Electronic circuits that a computer is assembled from/ The platform on which software executes
* Main memory: Memory that is directly accessible to the processor
* Memory location: a separately addressable area of main memory
* RAM: random access memory; volatile memory in which the locations can be accessed directly in any order
* ROM: read only memory; non-volatile memory that cannot be written to after it has been set up
* EEPROM: electrically erasable programmable read only memory; read only memory which's contents may be altered
* Computer Bus: a set of parallel wires connecting independent components of a computer
* System Bus: the main set of buses connecting the processor, main memory and I/O controllers, consisting of the data, control and address bus
* Peripheral:a computer device that is not part of the central processing unit
* I/O device:a hardware unit that sends or receives data or stores data, by communicating with the processor via a I/O controller
* I/O controller:an electronic circuit that connects to a system bus and an I/O device
* Secondary storage:permanent data storage not directly connected to the processor
* Memory address:an unique numeric code corresponding to a location in memory
* Stored program concept:a program must be present in main memory to be executed; it is processed by fetching machine code instructions in sequence from the main memory and executing these codes, one at a time in the processor
* Register:a very fast memory location inside the processor or I/O controller
* General purpose registers:a register that is not assigned a specific role
* Dedicated registers:a register assigned a specific role by the processor designer, e.g. the MBR or MAR
* Clock speed:the frequency in megahertz at which the processor executes instructions
* Word length:the number of digits in a binary word, e.g. 8-bits
* Bus width:the number of wires in a bus
* Machine code instruction:a binary code that a machine can understand
* Compiled high level language program:a program translated into machine code before it is executed on a computer
* Op-code:the part of a machine code instruction that denotes the basic machine operation, e.g. ADD
* Operand:the part of a machine code instruction that represents a single item of data used by the op-code
* Instruction set:the set of bit patterns or binary codes for the machine operations that a processor has been designed to perform
* Track:one of the concentric rings on a platter of a hard disk
* Sector:a subdivision of a track
* Disk block:the smallest unit of transfer between a computer and a disk; the size of a sector (size as in binary length)
* USB:Universal Serial Bus; allows peripherals to be connected using a standard interface socket
* Access time:the time the time between accesses on a storage device
* Software:a series of instructions that are executed by the hardware
* Assembler:translates assembly code into machine code
* Compiler:translates high level language code into object code
* Interpreter:analyses and executes high level language code line by line
* Bespoke software:written for a customer's specific need
* General purpose software:software that can be used for many different tasks
* Special-purpose software:software designed to support a specific task
* Internet:a network of computer networks and computers using unique IP addresses and communicate via the TCP/IP protocols
* Gateway:connects networks that use different link layer protocols
* Router:connects networks that use the same link layer protocol
* IP-Address:a unique address that identifies a computer or other device on a network
* WWW:World wide web; a system of interlinked hypertext documents accessed via the internet
* Domain name:a name that identifies one or more IP addresses
* FQDN:Fully Qualified Domain Name; it consists of a host ID and domain name
* DNS server:server that translates fully qualified domain names into IP addresses
* URL:Uniform Resource Locator; a URI that identifies a resource by its network location
* URI:Uniform Resource identifier; specifies how to access a resource on the internet
* Server:a software process that provides a service requested by a client
* Client:a software process that requests and uses the services provided by a server
* Client-Server model:a client software process initiates a request for a server software process to respond to