Structure of the CPU

• Central Processing Unit (CPU)


  The CPU is made up of many different components
• Arithmetic and Logic Unit (ALU)

  • Performs all of the arithmetic and logical operations of the CPU, including:

  Addition and subtraction, multiplication and division.

  Comparisons such as whether numbers are equal or if one is greater than another.

  Boolean operations such as AND, OR and NOT.
• Control Unit (CU)

  • Sends out control signals to other parts of the CPU.

  Executes program instructions by following the fetch-decode-execute cycle.

  Made up of two key parts:

  The clock - coordinates the CPU's activity.

  The decoder - decodes program instructions.
• Registers

  Tiny amounts of super-quick memory within the CPU used to hold information needed for the CPU to work. Each register holds a specific piece of information.
• Special purpose registers

  • Registers that have a predetermined purpose in the central processing unit
• Program Counter (PC)

  • Holds the memory location address of the next instruction to be performed by the CPU. The PC does not store the instruction itself.
• Accumulator (ACC)

  • Holds the result of calculations and operations performed by the arithmetic logic unit (ALU).
• Memory Address Register (MAR)


  • Holds the address of the location in memory where data is to be retrieved or stored.
• Memory Data Register (MDR)


  • Holds the data that has been retrieved from memory, or that is about to be stored in memory.
• Current Instruction Register (CIR)


  • Used to store the current instruction that the CPU is working on.
• Buses

  Used to send and receive data and instructions from main memory (RAM)
• Von Neumann CPU

  • Has 3 buses:

  The data bus

  The address bus

  The control bus
• Data bus

  • Retrieves data and instructions from main memory
• Address bus

  • Sends addresses to the main memory
• Control bus

  • Sends control signals to other devices inside the computer system