Computer Systems - Fetch-Execute Cycle

Question 1

What does a CPU do?

Answer 1

Carries out instructions - the Fetch-Execute cycle describes how it does it

Question 2

How long does each Fetch-Execute cycle take?

Answer 2

It depends on the processing power but usually, it takes multiple clock cycles

Question 3

What happens in the FETCH instruction of the cycle?

Answer 3

1. The Control Unit reads the memory address of the next CPU instruction
2. The instruction stored in that address is copied from memory to one of the registers
3. The memory address in the control unit is incremented to point to the address of the next instruction - so it gets ready for the next cycle

Question 4

What happens in the DECODE instruction of the cycle?

Answer 4

1. The instruction that was copied from memory, is decoded by the Control Unit
2. The Control Unit prepares for the next step - e.g loading other values into the registers

Question 5

What happens in the EXECUTE instruction?

Answer 5

The instruction is performed

Question 6

What are the different instructions that could be executed?

Answer 6

Load data from memory
Write data to memory
Do a calculation or logic operation (using the ALU)
Change the address in the control unit
Halt the program