Fetch, Decode, Execute Cycle

Question 1

What happens during the fetch stage

Answer 1

The PROGRAM COUNTER is checked, as it holds the address of the next instruction to be executed
The address is then copied into the MEMORY ADDRESS REGISTER.
The address is then sent along the ADDRESS BUS to main memory, where it waits to receive a signal from the CONTROL BUS
The CONTROL UNIT sends control signals along the CONTROL BUS.
The contents stored in memory can be sent along the DATA BUS to the MDR
The data received by the MDR from memory now gets copied into the CIR
The contents of the PC increment so the address it contains points to the next instruction to be executed

Question 2

What happens during the decode stage

Answer 2

The instruction held in the CIR is decoded by the CU ( Decode unit)
By decoding the instruction you can see the operation needed which will go into the CPU’S accumulator

Question 3

Opcode

Answer 3

What to do
Used to determine the type of instruction and what hardware to use to execute it

Question 4

Operand

Answer 4

What to do it to
Address of where the operation is performed

Question 5

What happens during the execute stage

Answer 5

Send address to the MEMORY ADDRESS REGISTER
CONTROL UNIT sends a control signal along the CONTROL BUS to RAM
Send the address down the ADDRESS BUS to main memory
Contents stored in memory can now be sent along the DATA BUS to the MEMORY DATA REGISTER
The contents of the MEMORY ADDRESS REGISTER are now copied to the ACCUMULATOR
The instruction is now complete

Question 6

Why does control bus go both directions?

Answer 6

Brings back confirmation of it working