Processor Components

• Arithmetic Logic Unit (ALU)


  The problem solving part of the processor
• Arithmetic Logic Unit (ALU)


  • Performs arithmetic operations
  • Performs logic operations
  • Performs shift operations
• Arithmetic operations


  • Add
  • Subtract
  • Multiply
  • Divide
• Logic operations


  • AND
  • OR
  • NOT
  • XOR
• Shift operations


  • Moving bits left
  • Moving bits right within a register
• Control unit


  Part of the processor that coordinates the activity of all other components
• Control unit function


  Controls the flow of data through the CPU by decoding instructions from memory and sending them to the appropriate functional units
• Control signals


  • Memory write
  • Memory read
  • Bus request
  • Bus grant
  • Clock
• Memory write


  Data from the data bus is written into the addressed location in RAM
• Memory read


  Data from the addressed location in RAM is placed on the data bus
• Bus request


  A device is requesting use of the data bus
• Bus grant


  The CPU has granted access to the data bus
• Clock


  Used to synchronise operations
• Control signals are sent along the control bus between the control unit and the other components of the computer
• Executing Instructions:

  Carrying out a sequence of programming instructions requires many different pieces of information to be held.

  The processor has to temporarily hold the current instruction being executed.

  It has to hold the address of the data that it needs, and also the data itself.

  It has to keep track of the address of the next instruction to be executed
• The Fetch-Decode-Execute cycle:

  Fetch:

  1. The address of the next instruction is copied from the PC to the MAR
  2. The instruction held at that address is copied to the MDR
  3. simultaneously, the contents of the program counter are incremented
  4. the contents of the MDR are copied to the CIR

  Decode:

  1. the instruction held in the CIR is decoded
  2. it is split into opcode and operand to determine the type of instruction it it. Additional data, if required is fetched from memory.
  3. ... and passed to the accumulator

  Execute:

  the instruction is executed and the result is held in the accumulator or stored in memory.
• The program counter:

  Holds the address of the next instruction to be executed
• The MAR (memory address register):

  A register that is located in the CPU it stores the address of the memory location currently in use.
• The MDR (memory data register):

  A register located in the CPU it stores data being transferred to and from the immediate access-store. Acts as a buffer, allowing the central processor and memory unit to act independently without being affected by minor difference in operation. A data item will be copied to the MDR ready for use at the next clock pulse, it can either be used by the central processor or be stored in main memory.