3.1.4 - Hardware

Central Components of a Computer System
1. Arithmetic Logic Unit (ALU)
  1. Performs two sort of operations on data:
    1. Arithmetic
      1. Addition
      2. Subtraction
      3. Multiplication
      4. Division
    2. Logical
      1. Comparison
      2. Less than?
      3. Greater than?
      4. Equal to?
    3. Acts as a conduit through which all I/O to the computer is done.
      1. Gateway to the processor.
2. Control Unit
  1. Co-ordinates and controls all the operations carried out by the computer.
  2. Operates by repeating three operations:
    1. 1. Fetch
      1. Causes the next instruction to be fetched from main memory.
    2. 2. Decode
      1. Produces signals which control other parts of the computer such as the ALU.
    3. 3. Execute
      1. Causes the next instruction to be executed.
3. Memory Unit (RAM)
  1. Quick and easily accessible information can be read from and written to by the Control Unit.
  2. Access to and from the Memory Unit is very fast.
  3. Normally contains:
    1. Operating System (OS)
    2. Software currently in use.
    3. Data which the software is using.
  4. Volatile
    1. Data is lost when the computer is shut down.
4. Registers
  1. An extremely fast piece of on-chip memory, for temporary storage.
  2. The registers you need to know are:
    1. Program Counter (PC)
      1. Holds the address of the next instruction to be executed.
    2. Memory Address Register (MAR)
      1. Holds the address in memory of the location containing:
        The next piece of data to be read.
        The next instruction to be used.
    3. Memory Data Register (MDR)
      1. The contents of the address specified in the MAR are copied to the MDR. This may be:
        An instruction/operation.
        Data to be used (with an instruction).
        Data to be copied to an address.
    4. Current Instruction Register (CIR)
      1. Holds the instruction while it is being decoded/executed.
      2. The contents of the MDR are copied to the CIR if it is an instruction.
      3. Contains the operator and the operand of the current instruction.
    5. Status Register
      1. Contains "status" bits or flags that are set or cleared based on the result of an instruction.
    6. Accumulator
      1. Works with the ALU to performs arithmetic functions.
      2. Temporary storage areas.
5. Buses
  1. A set of parallel wires connecting two or more components of the computer.
  2. There are three buses:
    1. Control Bus
      1. Carries command and control signals to and from every connected device.
      2. Fully bi-directional for all connected devices.
    2. Address Bus
      1. Communicates the physical addresses of computer memory elements/locations that the requesting device wants to access.
    3. Data Bus
      1. Transports the actual physical data to and from the memory.
      2. Both single-directional and bi-directional.
Device Connectivity
1. Hard-wire (Physical)
  1. Copper Coaxial
  2. Fibre Optic
  3. Copper Twisted Cable
2. Wireless (Non-physical)
  1. Wifi
  2. Satellite
  3. Bluetooth
3. Measured in the rate at which data can be transmitted.
  1. Baud Rate
4. Data rate can be increased by:
  1. Using higher quality cable.
    1. Less attenuation.
  2. Increasing frequency at which data is sent.
5. Each signal can be sent on a different frequency or wavelength.
  1. The number of frequencies that can be used before they interfere with each other is called the bandwidth.
    1. Copper has low bandwidth.
    2. Wireless has high bandwidth.
    3. Fibre optic has very high bandwidth.
Memory
1. Main Memory
  1. Random Access Memory (RAM)
    1. Volatile (Wiped when the computer is turned off).
    2. Used to store programs which are currently running.
    3. Stores data which is currently being processed.
  2. Read Only Memory (ROM)
    1. Non-Volatile
    2. Contents are permanently etched into the memory chip at the manufacturing stage.
2. Secondary Storage
  1. Magnetic
    1. Data recorded onto magnetic disks.
    2. Mechanical parts move over the disk's surface to read and write data.
    3. Advantages
      1. Cheap.
      2. Large storage capacity.
    4. Disadvantages
      1. Not portable.
      2. Durability can be an issue.
  2. Optical
    1. Data is recorded onto optical disks.
    2. Lasers read and write data.
    3. Advantages
      1. Highly portable.
      2. Cheap.
    4. Disadvantages
      1. Low storage capacity.
      2. Easily damaged.
  3. Solid State
    1. Data is recorded onto solid state memory chips.
    2. Disadvantages
      1. Expensive.
      2. Less storage capacity than magnetic storage.
    3. No mechanical moving parts.
    4. Advantages
      1. Fast.
      2. Durable.
Transfer of Data
1. Fetch/Decode/Execute Cycle
2. Interrupts
  1. A signal from some device or source seeking the attention of the processor.
  2. The interrupt signal is sent along the control line to the processor.
    1. The currently executing program is suspended and control is passed to an ISR (Interrupt Service Routine).
  3. There is a register in the CPU called the Interrupt Register.
    1. It is checked at the beginning of every F/D/E Cycle.
3. Buffers (Cache)
  1. Used to help match the speeds of many devices.
  2. Used between the high processing speeds of the CPU and the slower speeds of the Main Memory.
  3. Data can be written to a buffer as though it were Main Memory.
    1. The high speed processor can then carry out the next operation whilst the Main Memory reads from the buffer instead.

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