Architecture of the CPU

• Memory Address Register

  Stores the memory address which will be accessed next.
• Memory Data Register


  Stores the data that has just been fetched from, or the data that is about to be written to, the memory.
• Program Counter


  Stores the memory address of the next command to be executed
• Accumulator

  Stores the result of calculations.
• Explain the purpose of CPU registers

  The registers are super fast memory that store tiny amounts of data or instructions that the CPU can access extremely quickly.
• Control Unit (CU)

  • The control unit is in overall control of the CPU (by following the fetch-execute cycle)
  • It controls the flow of data inside the CPU and outside the CPU.
• Arithmetic Logic Unit (ALU)

  • Does all the calculations
  • Performs logic operations.
  • Contains the accumulator register
• Cache

  • Very fast memory in the CPU
  • Stores regularly used data so the CPU can access it quickly
  • Very low capacity
  • Expensive compared to RAM and secondary storage
• Registers

  • Memory Address Register (MAR)
  • Memory Data Register (MDR)
  • Program Counter (PC)
  • Accumulator
• What will happen if the program counter is not incremented in each cycle?

  The memory address of the next instruction will always be the same, so the CPU would carry out the same instruction repeatedly.
• Fetch

  • The instruction stored at the memory address in the PC is fetched
  • The memory address is copied to the MAR
  • The memory contents at the address are fetched into the MDR.
  • The PC is now incremented

  Decode

  • The instruction is copied from the MDR to the CU
  • The fetched instruction is decoded by the CU

  Execute

  • The decoded instructions is executed
  • The cycle is repeated