Architecture of CPU

• How does CPU carry out instructions?

  Fetch - Execute Cycle
• Fetch Instructions

  Copy memory address from program counter to MAR

  Copy the instruction stored in the MAR address to the MDR

  Increase the program counter to point to the address of the next instruction, ready for the next cycle
• Decode Instruction


  The instruction in the MDR is decoded by the CU

  The CU may then prepare for the next step
• Execute Instruction


  The instruction is performed - (e.g. load data from memory, write data to memory, do a calculation, change the address in the PC or halt the program)
• Common CPU components


  Control Unit

  Arithmetic Logic Unit (ALU)

  Cache

  Registers
• Control Unit


  Main job is to manage fetching, decoding and execution of program instructions by following the ‘fetch-execute cycle’

  It controls the flow of data inside CPU (registers, ALU, cache etc.) and outside CPU (main memory and input/output device)
• Arithmetic Logic Unit (ALU)


  does all the calculations

  completes simple addition and subtraction

  compares size of numbers and can do multiplications and divisions using repeated addition and subtraction

  performs logic operations such as AND, OR and NOT and binary shifts

  contains accumulator register
• The Cache


  Very fast memory in CPU (slower than registers but faster than RAM)

  Stores regularly used data so CPU can access it quickly

  (when CPU requests data, cache is checked first, if not it will be fetched from RAM)

  Caches have low capacity and is expensive compared to RAM/secondary storage
• Different Levels of Cache

  L1 (quickest but lowest capacity)

  L2 (slower than L1 but can hold more)

  L3 (slower than L2 but can hold more)

  
• Registers

  temporarily hold small amounts of data needed by the CPU

  they are very quick to read/write (quickest memory)
• Von Neumann’s Design

  main features of the design is that it only uses one memory for both data and instructions
• Components in Von Neumann's Design

  Program Counter (PC)

  Accumulator

  Memory

  Memory Data Register (MDR)

  Memory Address Register (MAR)
• Program Counter (PC)


  holds the memory address of the instruction for each cycle
• Accumulator


  stores intermediate results of calculations in the ALU
• Memory


  holds the program instructions and the program data
• Memory Data Register (MDR)


  Holds the actual data or instruction

  This may be fetched from Memory or be waiting to be written to Memory
• Memory Address Register (MAR)


  Holds any memory address about to be used by the CPU

  The address might point to data or a CPU instruction