Computer Science OCR GCSE

• CPU - Central Processing Unit

  executes instructions, processes data, fetches data and stores data into main memory once executed (performs the fetch decode execute cycle). It make a computer system function
• MAR - Memory Adress Register


  holds the address of the current instruction that is to be fetched from memory
• MDR - Memory Data Register


  holds the contents found at the address held in the MAR
• Main Memory/Primary Storage


  the component of the computer that holds data, programs and instructions that are currently in use
• Address Bus


  carries memory addresses for locations to be read from or written to
• Data Bus


  carries data between the CPU and main memory
• Control Bus


  carries control signals around the CPU and memory
• ACC - Accumulator


  holds the data being processed and the results of processing
• PC - Program Counter


  holds the memory address of the next instruction to be fetched from main memory
• ALU - Arithmetic Logic Unit


  performs arithmetic (add/subtract) and logical (AND, OR) operations
• CU - Control Unit

  Follows the fetch decode execute cycle and controls the flow of data within the CPU..
• Cache


  a small amount of memory which temporarily holds instructions and data that the CPU is likely to reuse. It is very fast and sits between the CPU and main memory. It is faster to access than RAM. The bigger the cache, the less time a processor has to wait for instructions to be fetched.
• RAM - Random Access Memory


  is a short term data storage that holds data and instructions that are currently in use by the processor, is volatile (loses power when turned off)
• ROM - Read Only Memory


  permanent instructions that cannot be changed, non-volatile. It allows a computer to start up or regenerate each time it is turned on
• I/O Controllers


  used to allow an interface between a hardware device external to the motherboard (e.g. a keyboard) and the processor itself
• System Buses


  a set of parallel wires connecting two or more independent components of a computer system in order to pass signals between them.
• Clock Speed


  the operating speed of the computer/speed of the fetch-decode-execute cycle, measured in hertz (1GHz processor executes 1 billion instructions per second/executes 1 billion cycles per second)
• Embedded System


  a computer system that is made up of both hardware and software, usually used for very specialised tasks, have no UI (user interface), they do the same task repeatedly
• Boot Sequence


  a sequence of processes that contains all the information and instructions to get the computer up and running
• Secondary Storage


  devices such as a CD or USB pen that is non-volatile and stores data and programs for later use. They are external to the computer.
• Registers


  storage locations that temporarily holds data and instructions that the CPU can quickly access
• CIR - Current Instruction Register


  holds the instruction that is about to be decoded or executed
• Virtual Memory


  an area of secondary storage that the computer uses to store the contents of RAM temporarily when there isnt enough space in the RAM to store data so it only happens when the RAM is full. The computer will copy the items not currently in use from the RAM to virtual memory, freeing up space in RAM. Is volatile. The VM swaps data with RAM so its slow.
• Metadata


  extra data stored in a file which gives information about the file's properties (file format, resolution, colour depth, dimension, etc.)
• Sample Rate


  the number of samples recorded in any given period of time
• Duration


  how many seconds of audio the sound file contains
• Bit Depth


  number of bits available to store each sample (e.g. 16-bit)
• Bit rate


  a measure of how much data is processed for each second of sound (sample rate x bit depth)
• Compression


  the proccess of encoding data so that it needs fewer bits/bytes
• Sampling


  Height/amplitude of waveform is sampled/measured and converted to binary. Samples are taken at a regular intervals
• Lossy Compression


  compresses data files but loses some of the information, is irreversiblecant be done on a word document/text file. It reduces the colour depth of an image or the bit depth of an audio
• Lossless Compression


  compresses files without losing any information.Is reversible. One method of lossless compression is run length encoding (RLE). RLE looks at the data in a file for consecutive runs of the same data. These runs are stored as one item of data, instead of many. (e.g. 000111 being stored as 3031)
• Decomposition


  breaking a complex problem down into smaller problems to make things easier
• Abstraction


  removing irrelevant details from a problem to reduce complexity so you can focus on the important detail
• Input Sanitisation


  cleaning up input data/removing unwanted data (e.g. special characters)
• Authentication


  ensuring only allowed/authorised users can gain access (e.g. usernames/passwords)
• Validation


  checking wether input data should be allowed/is sensible (e.g. goals cannot be less than 0)
• Verification


  checking wether data has been entered correctly (e.g. double entry)
• Casting


  converting one data type to another, for example converting an integer to a real number
• Iterative Testing


  repeatedly testing while making changes/while the program is being developed