THE FDE cycle

Question 1

Describe what happens in Fetch phase

Answer 1

1. The addres of the next instruction is copied from the PC to the MAR.
2. The instruction held at that adressed location is copied to the MDR. Simultaneously, the contents of the PC are incremented so that it can hold the address of the next instruction.
3. The contents of the MDR are copie into the CIR.

Question 2

Describe what happens in Decode phase

Answer 2

The instruction held in the CIR is decoded. It is split into opcode and operand, where the opcode is used to determine what type of instruction and what hardware to use to execute it. The operand, on the other hand, holds eitheir:

• the address of the data to be used with the operation , which is copied into the MAR, or
• the actual data to be operated on, which will be copied to the MDR, or
• the data to be operated on may be passed to the accumulator/ALU

Question 3

Describe what happens in Execute phase

Answer 3

The appropriate instruction/opcode is carried out on the operand.