Instruction Execution - week 7

Question 1

What are the three main phases of the machine cycle?

Answer 1

Fetch, Decode, and Execute.

Question 2

What happens in the Fetch phase

Answer 2

The CPU brings the instruction from store to the instruction register.

Question 3

What happens in the Decode phase?

Answer 3

The instruction is interpreted (decoded) to determine what operation is to be performed and how the operands are to be accessed.

Question 4

What happens in the Execute phase

Answer 4

Decoding the instruction, accessing the operands (if any) and carrying out the operation.

Question 5

Which phase is the same for all instructions and why?

Answer 5

The Fetch phase is the same for all instructions because every instruction must first be retrieved from memory in exactly the same way.

Question 6

Which phase varies from instruction to instruction?

Answer 6

The Execution phase varies, depending upon the operation.

Question 7

How is a program stored in memory?

Answer 7

A program is a sequence of instructions stored in store/memory/RAM.

Question 8

What are the two parts of an instruction?

Answer 8

An Operation and an Operand.

Question 9

What does the Operand part specify?

Answer 9

It specifies what the operation acts on (e.g. a value, a register, or a memory address).

Question 10

What are the control lines used for?

Answer 10

To carry control signals such as read and write between CPU and memory.

Question 11

Which two registers are inside the Control Unit in the final model for instruction access?

Answer 11

IR and PC.

Question 12

How does the Control Unit access an instruction?

Answer 12

The Program Counter (PC) holds the address of the next instruction

The Control Unit sends that address to memory

Memory returns the instruction stored at that address

The instruction is read into the Instruction Register (IR)

Question 13

What is the role of the store in this model?

Answer 13

The store holds instructions and data and is accessed via read/write data operations.

Question 14

List the four bullet points for “To carry out an instruction”.

Answer 14

it must first be fetched from store to the control unit.

then interpreted (or decoded) to determine what operation is to be performed and how the operands are to be accessed.

the operands must be fetched from store to the ALU

the operation upon them must be executed, using control signals supplied by the control unit.

Question 15

What is the “instruction cycle”

Answer 15

Fetch phase: bringing the instruction from store to the instruction register.

Execution phase: decoding the instruction, accessing the operands (if any) and carrying out the operation.

Question 16

What does PC do at the start of a fetch phase

Answer 16

“At the start of a fetch phase, PC points to the next instruction to be executed.”

Question 17

What are the 4 stages of the fetch phase

Answer 17

1. Contents of the PC are placed in the MA: (this sets the store location of the next instruction.)
2. Contents of the cell at the address in MA are placed in the MB: (this gets the contents of the cell)
3. Contents of the MB are placed in the IR: (the instruction is now ready for decoding.)
4. Increment PC: (so that the next instruction in sequence will be accessed next.)

Question 18

Describe the stages of execution

Answer 18

The address part of the instruction is copied to the MA

The contents of the memory location at that address are copied into MB

The Control Unit performs the required operation

The result is stored in a register or memory location.

Question 19

What does ADD R0, (20) mean

Answer 19

Add the contents of location (address) 20 to register R0.