1. The Fetch – Execute Cycle
The control unit sets up the address bus by transferring the contents of the PC to the MAR
The control unit activates the READ line on the control bus and the contents of the memory location are transferred to the MDR
The control unit decodes and executes the instruction
The instruction is transferred to the IR in the control unit
The PC is incremented

2. Memory Read Operation
The control unit sets up the address bus by transferring the memory address to be accessed to the MAR
The control unit activates the READ line on the control bus and the contents of the memory location are transferred to the MDR
If necessary the contents of the MDR are transferred to the Accumulator

3. Memory Write Operation
The control unit sets up the address bus by transferring the memory address to be accessed to the MAR
Set up data bus by transferring data to the MDR
The control unit activates the WRITE line on the control bus and the contents of the MDR are transferred to the memory location