Registers And The F-E Cycle

Question 1

What does the Program Counter do?

Answer 1

• A register that tracks the RAM address of the next instruction to be executed.

Question 2

What does the Memory Address Register do?

Answer 2

• A register that tracks the RAM address of the instruction that is to be fetched.

Question 3

What does the Memory Data Register do?

Answer 3

• Stores the instruction that has been transferred from RAM to the CPU.

Question 4

What does the Current Instruction Register do?

Answer 4

• A register that stores the instruction that has been fetched from RAM, and is about to be decoded or executed.

Question 5

What does the accumulator do?

Answer 5

• Stores the result of mathematical or logical calculations.

Question 6

Describe each stage of the Fetch-Execute Cycle (7/8 marks)

Answer 6

• 1. The PC displays the address in RAM of the next instruction to be executed. This value is copied into the MAR.
• 2. The PC is increased by 1. This prepares the CPU for the next instruction to be fetched.
• 3. The CPU checks the address in RAM which matches the address held in the MAR.
• 4. The instruction in RAM is transferred to the MDR.
• 5. The instruction in the MDR is copied into the CIR.
• 6. The instruction in the CIR is decoded and executed. Any result of an execution is stored in the ACC.
• 7. The cycle repeats by returning to the first step and checking the program counter for the address of the next instruction.