Computer Science

• Fetch: fetches the next instruction from the RAM (Main memory), and then brings the instruction back to the CPU
• Decode: Works out the instruction from the binary code and executes it.
• Execute: Carries the instruction out, like going back to the RAM to grab data, preforming calculations and storing information back to the RAM
• CPU consists of the ALU, CU, cache and registers
• ALU: Arithmetic Logic Unit - preforms calculations and logical decisions
• CU: Control Unit - sends signals to control how data moves around the CPU  Controls the flow of data between the CPU and the RAM
• Cache: Provides fast access to frequently used instructions and data
• Registers: Tiny, super fast pieces of onboard memory inside the CPU each with a very specific purpose
• Von Neumann Architecture


  consists of a CU, ALU, Memory Unit and inputs and outputs. it is based on the concept of the stored program concept. Both instruction data and program data are stored in the same memory in binary form. There is no way to know if the pure binary is held in memory that is representing instructions or data simply by looking at it
• PC: Program Counter - holds the address of the next instruction to be executed.
• MAR: Memory Address Register - holds the address where data is to be fetched/stored in memory
• MDR: Memory Data Register - holds the data fetched from, or to be written to memory
• Accumulator: holds the result of calculations
• Clock Speed: measured in Hertz. Number of cycles per second, 3.2GHz can be fetched per second.
• Cache Size: Temporary storage of data and instructions being written from the RAM, stores copies of frequently used instructions. its much quicker to fetch data from the cache rather than going to the RAM
• Number of Cores: cores are a complete copy of the CPU. Doubling the number of cores doesn't simply increase speed. The CPU has to communicate with each other, so it takes more time
• Embedded System: a computer system with a dedicated function within a larger mechanical system

  Properties:

  1. low power consumption
  2. small size
  3. low cost per unit

  Examples:

  • traffic lights
  • hospital equipment
• Primary Storage: holds data and instructions which the CPU needs access to while a computer is running. The CPU can access data from the primary storage, rather than the secondary storage
• Primary storage consists of: RAM, ROM, Registers and Cache
• RAM: Random Access Memory - Holds the operating system, programs and data, in use by the CPU when the computer is running. Read and Write. Large in comparison to ROM
• ROM: Read Only Memory - Holds the first instructions for when the computer is first turned on (Boots strap). Non-volatile, read only, small in comparison to RAM
• Virtual Memory: needed when there is not enough physical RAM to store the open programs, its held on the hard disk. Programs are transferred from RAM to virtual memory when not being used. then transferred back to RAM when in use
• Secondary Storage: needed because ROM is read only and RAM is volatile. its needed to store programs and data when the power is turned off, semi permanent storage of data that can change, back up of data files
• optical: Low capacity compared to other types of storage, slow access to data, prone to scratching , thin, lightweight and portable
• Magnetic: high storage capacity, quick access to data, has moving parts which eventually fail, hard disks preform better if they are defragmented
• Solid State: medium storage capacity, very quick access to data, no moving parts so very reliable, no noise, low power, limited number of read/write cycles, expensive
• defragmented: reorganising data on the hard disk
• capacity: how much data needs to be stored
• speed: how quickly can data be read and transferred
• portability: if data needs to be transported, are size, shape and weight important
• durability: will the media be damaged by shocks and in extreme conditions
• Reliability: Does it need to be used over and over again withput failing
• cost: how expensive is the media per byte of storage
• optical:

  • suitable for read only data distribution on a large scale
  • suitable for small capacity situations
• magnetics

  • suitable when very high data capacity is required
  • suitable for very fast access data
  • suitable for low cost situations
  • suitable for cloud storage on server farms
• magnetic

  • suitable when very high data capacity is required
  • suitable for very fast access data
  • suitable for low cost situations
  • suitable for cloud storage on server farms
• solid state:

  • suitable for low power systems
  • suitable for portable devices
  • suitable for small-medium data capacity requirements
  • suitable for silent operations
  • suitable for very fast access to data
  • suitable in situations when the device needs to be small and lightweight
• character set: a defined list of characters that is recognised by the computer. each number is represented by a unique binary
• well known character sets:

  • ASCII - 7 bit character set with $2^7$ characters = 128
  • extended ASCII - 8 bit character set with $2^8$ = 256
  • UNICODE - 24 character set with 16 million
• bitmap: a method of storing images using pixels/binary