1. Fetch-Execute cycle

2. Memory Read operation Read from memory

3. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 The Program Counter (PC) holds the address of the next instruction

4. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 The contents of the PC are copied to the Memory Address Register (MAR) 10110111

5. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 Memory address to be accessed is placed onto the address bus 10110111

6. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 The control bus sends a read signal >>read>>

7. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 The contents of the memory location are passed along the data bus to the Memory Data Register (MDR) 01010111

8. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 1 The PC is incremented ready for the next instruction 2

9. Memory Write operation Write to memory

10. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 2 Data is placed onto the data bus 1110011

11. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 2 Memory address to be written to is placed onto the address bus 11110010

12. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 2 A write signal is sent along the control bus >>write>>

13. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 2 The data is transferred to the destination in memory 10111110

14. Main Memory MAR MDR Control Unit ALU Other registers Electronic Clock Clock pulses Address bus Data bus Control bus PC 2 The PC is incremented 3