FDE cycle

Question 1

What needs to be done before the FDE cycle?

Answer 1

Instructions need to be translated into machine code, then loaded into the RAM before starting.

Question 2

What happens in step 1 of the fetch stage?

Answer 2

• PC keeps address of next instruction
• PC contents copied into MAR
• Address of next instruction is placed on address bus

Question 3

What happens in step 2 of the fetch stage?

Answer 3

• Control unit instructs memory read operation of the current address through the data bus to the MBR
• PC incremented by 1

Question 4

What happens in step 3 of the fetch stage?

Answer 4

• Contents of MBR is copied to CIR

Question 5

What happens in the decode stage?

Answer 5

• Control unit decodes instruction that is kept in CIR

Question 6

What happens in the execute stage?

Answer 6

• Instruction is executed
• Depending on the type of instruction, executing the instruction can involve the ALU and the general purpose registers, the accumulator or the main memory