The CPU & Fetch-Execute Cycle

COMP124 Lectures 2021-02-15

The CPU

This is a block diagram of a CPU:

• CU - Control Unit
  • Fetches instructions and works out what to do with them.
  • Controls the flow of execution.
• ALU - Arithmetic Logic Unit
  • Performs operations on bit patterns in order to complete mathematical and logic operations.
• Registers
  • Memory on the CPU die.
  • General purpose registers
    • A,B,C,D
  • IR - Instruction Register
    • Always holds the address of the next instruction being executed.
  • IP - Instruction Pointer
    • Contain memory addresses.
• Memory Interface
  • MAR - Memory Address Register
    • Stores the address of an area of memory to be addressed.
  • MDR - Memory Data Register
    • Stores a word of memory to/from the RAM.

The Fetch-Execute Cycle

The control unit continually direct the fetching and execution of instructions in a cycle.

1. Copy address contained in the IP to the MAR. Issue read request to memory.
2. Increment IP to point to next instruction. (Add word size e.g. 4/8)
3. Instruction will arrive in MDR. Copy it to IR.
4. Decode IR to work out what is required.
5. Fetch any operands.
6. Carry out execution.
7. Go to step 1.

Steps 5 and 6 may cause further memory accesses.