1.1.1 Architecture of the CPU

• Central Processing Unit (CPU)


  The main part of the computer (the brain), consisting of the registers, ALU and control unit
• Fetch-execute cycle

  Retrieving an instruction from store, decoding it and carrying it out
• Arithmetic Logic Unit (ALU)


  Performs calculations e.g. x = 2 + 3 and logical comparisons e.g. IF x > 3 in the CPU
• Control Unit (CU)


  Decodes instructions. Sends signals to control how data moves around the CPU
• Cache

  Memory in the processor providing fast access to frequently used instructions and data
• Register

  Tiny areas of extremely fast memory located in the CPU normally designed for a specific purpose, where data or control information is stored temporarily e.g. the MAR, MDR etc.
• Von Neumann architecture

  Traditional computer architecture that forms the basis of most digital computer systems. Instructions are fetched, decoded and executed one at a time
• Memory Address Register (MAR)


  Holds the address of data ready for use by the memory data register, or the address of an instruction passed from the program counter. Step 2 of the fetch, decode, execute cycle
• Memory Data Register (MDR)


  Holds the data fetched from or to be written to the memory. Step 3 of the fetch, decode, execute cycle
• Program Counter(PC)


  Holds the address of the next instruction to be executed. Step 1 of the fetch, decode, execute cycle
• Accumulator

  Holds the result of calculations
• Arithmetic Logic Unit (ALU)


  Performs operations on data e.g. Addition, subtraction
• Control Unit (CU)


  Coordinating activities of the CPU
• Registers

  Quick, small stores of data within the CPU
• Memory Address Register (MAR)


  Holds memory address for data or an instruction about to be used by the CPU
• Memory Data Register (MDR)


  Holds actual data or instruction
• Accumulator

  Stores results of calculations in the ALU
• Program Counter (PC)


  Holds memory address of the instruction for each cycle
• Fetch

  The next instruction is retrieved by CPU from main memory
• Decode


  The instruction is broken down and decided so computer can understand
• Execute


  The CPU performs what the instructions told
• Buses

  Collection of wires that carry signals between various components of the computer system