Computer science

• CPU purpose

  To carry out the processing of data on the computing system and to perform the FDE cycle
• Affects CPU performance - clock speed


  number ofFDE cycles per second (measured in Hz - hertz)
• Affects CPU performance - Cache size


  high speed memory CPU uses (similar speed to CPU - faster than RAM) - checked for instructions that have been fetched before - faster to fetch instructions from larger cache = more space or instructions
• Affects CPU performance - number Of cores


  Number of independent processor in CPU working together. Higher number of cores = better performance as it can multitask BUT TAKES TIME FOR THEM TO COMMUNICATE
• ALU


  arithmetic logic unit - temporarily stores arithmetic and logic results
• MDR


  memory data register - stores actual instructions or data
• MAR


  Memory address register - stores address of instruction/data
• CIR


  Current instruction register - holds current instruction to be executed
• PC


  program counter - points to next instruction
• Steps in cycle - Fetch


  Address copied from PC to MAR. Instruction is copied. From MAR to MDR. instruction copied from MDR to CIR. Increment PC
• Steps in cycle - decode

  instruction is performed. ALU may be used for logic or calculations. ACC (accumulator) stores result
• What does moore’s law say 


  principle that states that the number of transistors on a microchip will double every two years
• what is. Von Newman architecture

  design upon which many general purpose computers are based
• Registers definition


  small amounts of high-speed memory contained within the CPU
• define embedded systems with 3 examples

  Processor built into another device (e.g. dishwasher, microwave, smart phone)
• Embedded systems purpose 


  to control the device and to allow a user to interact with it And help by being specialised to certain things (also it doesn’t contain an operating system)
• Primary storage


  holds data and programs that are currently in use (E.g. RAM and ROM)
• RAM (Random access memory)

  Stores data and program currently being used. Can be changed any time Volatile memory (data lost when power Is turned off) Large memory. More RAM = more programs the can run at the same time
• ROM (read only memory)


  stores BIOS (stores instructions to start up. the computer). programmed during computers manufacture (can’t usually be changed). non-volatile (data not lost when power turned off).
• Flash memory


  Non-volatile, can be changed, don’t need power supply, no moving part, fast, reliable
• types of flash memory


  memory cards, SD cards, USB sticks, SSD (solid state drives)
• Virtual memory


  extension to RAM. if not enough RAM then it will use some of the hard drive. access speeds from hard drive = slower than ram. more ram. = less need for virtual memory improving performance.
• Disk trashing

  computer 'freezes' due to pages being swapped too frequently between the hard disk and main memory. This issue becomes progressively worse as virtual memory is filled up.
• Secondary storage

  Stores files and programs when they’re not in use. It is long-term and non-volatile.
• type of secondary storage - optical

  lasers write data, good capacity, low cost, light, portable, easily damaged, slow access speed (e.g. DVD, CD)
• type of secondary storage - Magnetic

  Used to back up, high capacity, cheap, reliable, slow to read due to moving parts (e.g. hard disks, tapes)- using. Magnetized particles on spinning disk or tape
• type of secondary storage - Solid state


  No moving parts, very fast access speeds, large capacity but less than magnetic tape, more expensive, portable, reliable, not affected by being moved around (e.g. USB sticks, memory cards) - uses memory cells to store data
• Secondary storage - access speed (slowest to fastes)

  Optical disk, magnetic tape, HDD, USB/flash, SDD
• Secondary storage - cost (cheap to expensive)

  magnetic tape, optical disk, HDD, USB/flash, SDD
• Secondary storage - capacity (small to large)

  optical disk, USB/flash, SDD, HDD, magnetic tape
• Secondary storage - portability (difficult to easy)

  SDD, HDD, magnetic tape, optical disk, USB/flash
• Secondary storage - durability/ reliability (low to high)


  optical disk, magnetic tape, USB/flash, SDD, HDD
• Order of units


  Bits, nibble, byte, kilo, mega, giga, tera, peta
• Image file data capacity formula


  Width × height × colour depth
• sound file data capacity formula

  bits per sample × samples per second × bit depth
• Relative capacity of. Common files


  Text (small file size). JPEG (image) (medium size). MP3 (audio)(medium). PDF. MP4 (video)(large)
• Advantages of networks 


  Communication between workers. Allows sharing of data. Computers upgraded easily.
• WAN (wide area network)


  geographically remote, not owned by company
• LAN (local area network)


  small geographic area (one site), owned by organisation that uses it
• What affects performance of networks


  Bandwidth = amount of data transferred in a given time. Too man users can slow it down. Wired connections = more secure and faster than wireless. Hardware used to set it up can affect it. Network topology used