Fetch-execute cycle

• Processors operate in defined stages that are used to carry out program instructions

• The program is repeated over and over again for each instruction in a program

Step 1. The address of the next instruction is copied from the PC to the Memory Adderss Register (MAR)

Step 2. The instruction held at that address is copied to the Memory Data Register (MDR)

Step 3. Simultaneously, the contents of the Program Counter (PC) are incremented

Step 4. The contents of the MDR are copied to the current Instruction Register (CIR)

Step 5. The instruction held in the CIR is decoded

Step 6. It is split into OPERAND and OPCODE to determine the instruction type.

Step 7. Additional data can be fetched from the memory and is passed to the accumulator

• OPCODE - specifies the operation that is carried out

• OPERAND - either holds the address of the data to be used, which is then copied to MAR

  Or

  it holds the actual data to be operated on, which is passed onto the MDR

Step 8. Instructions are executed and the result is either held in accumulator or stored in memory.