Fetch decode execute cycle

from the vle online textbook

Buses

Communication channels through which data can be sent around the computer

Data bus

Carries data between the processor and memory

Address bus

Carries the address of the memory location being read from or written to.

Control bus

Sends control signals from the control unit.

Describe the purpose of the control unit

Fetch

• Contents of the PC are copied to MAR

• The read signal is sent across the control bus and the contents of the MAR are sent across the address bus.

• The contents of the memory location stored in the MAR are then sent across the data bus and stored in the MDR

• The contents of the MDR are then copied to the CIR

• The PC is incremented by one.

Decode

• the contents of the CIR are sent to the control unit

• the control unit decodes the instruction

execute

• the registers can be changed in different ways during the execution phase, depending on the instruction.

• if the instruction is for a memory location to be read from or written to (LDA or STA), then the address stored within the instruction will be loaded into the MAR.

• with STA the data stored in the ACC is sent to memory

• LDA the data is loaded from memory into the ACC

• If the instruction is to carry out a calculation, the contents of the MDR and ACC are sent to the ALU and the result sent back to the ACC.