Fetch-Decode-Execute cycle

Question 1

What are the 2 buses?

Answer 1

Address bus - takes the memory address to the RAM, finds the data that corresponds to the address and then the data is taken back on the data bus

Data bus - takes the data back to the memory address register from the RAM

Question 2

What are the 5 steps of the fetch stage?

Answer 2

Step 1 - The program counter copies the address of the next instruction it contains into the memory address register
Step 2 - The memory address register places the address to be used on the address bus
Step 3 - The memory address register triggers a ‘read’ signal that causes main memory (RAM) to place the instruction being asked for onto the data bus
Step 4 - The instruction on the data bus is loaded into the memory data register (also called memory buffer register)
Step 5 - The memory data register copies the instruction into the instruction register

Question 3

What happens during the decode stage?

Answer 3

The CPU decodes the instruction and prepares various areas within the chip in readiness of the next step

Question 4

What happens during the execute stage?

Answer 4

Data processing takes place, The instruction is carried out upon the data (executed). The result of this processing is stored in yet another register.

Question 5

What is the 1st step of the fetch stage?

Answer 5

Step 1 - The program counter copies the address of the next instruction it contains into the memory address register

Question 6

What is the 2nd step of the fetch stage?

Answer 6

Step 2 - The memory address register places the address to be used on the address bus

Question 7

What is the 3rd step of the fetch stage?

Answer 7

Step 3 - The memory address register triggers a ‘read’ signal that causes main memory (RAM) to place the instruction being asked for onto the data bus

Question 8

What is the 4th step of the fetch stage?

Answer 8

Step 4 - The instruction on the data bus is loaded into the memory data register (also called memory buffer register)

Question 9

What is the 5th step of the fetch stage?

Answer 9

Step 5 - The memory data register copies the instruction into the instruction register