[1.1.1] | 2: The FDE Cycle

Question 1

Describe the Fetch process of the CU.

Answer 1

1. PC copies address of next instruction into the MAR.
2. CU loads address onto address bus.
3. CU send a read signal along the control bus, causing the RAM to send the desired instruction down the data bus.
4. Instruction from the data bus is placed on the MDR.
5. MDR copies the instruction to the CIR
6. PC is incremented to point to the next instruction.

Question 2

Describe the decode process of the CU.

Answer 2

• The instruction is identified in the CIR and decoded. (Using the decoder’s instruction set.)

Question 3

Describe the execute process of the CU.

Answer 3

• The instruction in the CIR is carried out. (It may use values from the ACC, ALU or from memory.)

Question 4

What does the LDA command do? (Assembly)

Answer 4

Loads a value into the ACC from a specified memory location.

Question 5

What does the STA command do? (Assembly)

Answer 5

Stores a value from the ACC into a memory location.

Question 6

What does the ADD command do? (Assembly)

Answer 6

Add a value from memory to the total in the ACC.

Question 7

What does the HLT command do? (Assembly)

Answer 7

Halts the program.

Question 8

What does the DAT command do? (Assembly)

Answer 8

Labels a memory location with an identifier.