Fetch Decode Execute Cycle

Question 1

What happens in the “Fetch” stage

Answer 1

• PC contains the address of the next instruction
• It is then copied into the MAR
• It is then sent on the Address Bus, it then waits for a signal from the Control Bus
• the CU sends a signal on the Control bus saying memory read
• the contents of the address can now be transferred on the data bus
• the value of the program counter increases

Question 2

Decode

Answer 2

• The instruction in the CIR is decoded by the decode unit (it holds the opcode and the operand)
• The instruction (once translated) is moved into the MAR

Question 3

Execute

Answer 3

• The address is sent from the MAR to the RAM on the address bus
• The control bus will then send a read memory signal
• The data bus will then carry the address and memory to the MDR
• The value is then passed to the accumulator