Fetch - Decode - Execute Cycle

Question 1

What happens in the FETCH?

Answer 1

PC holds address of next instruction.
Address is copied to MAR
Location is sent down address bus and fetch signal is sent by control bus to get contents from the location
Contents from memory go to processor (using a data bus into MDR)
Contents in MDR and then copied to CIR
PC increments by 1

Question 2

What happens to the instruction within the CIR/?

Answer 2

it is split into an operand and opcode (using decode unit)

Question 3

What happens in DECODE?

Answer 3

• the decode unit is used to decode data from CIR into operand and opcode and store in CIR
• Check is preformed to make sure data is present for current instruction (in MDR)

Question 4

What happens in the EXECUTE?

Answer 4

Instruction address copied from CIR to MAR
Processor carries out instruction.
Return result into accumulator
Results sent to RAM
- please notice how PC value can change due to the statements