1.1 Processor Components

Question 1

Control Unit

Answer 1

The part of the CPU that coordinates the activity of all other components

Question 2

What are control signals?

Answer 2

Signals that are sent along the control bus between the control unit and the other components of the computer

Question 3

What is a bus?

Answer 3

Consist of a series of connectors that transfer signals between internal components

Question 4

System bus

Answer 4

Consists of three separate buses carrying control signals, addresses and data

Question 5

List some control signals:

Answer 5

• Memory Read
• Memory write
• Bus request
• Bus grants
• Clock

Question 6

Memory Read

Answer 6

Causes data from the addressed location in RAM to be placed on the data bus

Question 7

Memory write

Answer 7

Causes data on the data bus to be written into the addressed location in RAM

Question 8

Bus Request

Answer 8

Indicates that a device is requesting use of the data bus

Question 9

Bus grant

Answer 9

Indicates that the CPU has granted access to the data bus

Question 10

Clock

Answer 10

Used to synchronise operations

Question 11

What does the ALU do?

Answer 11

• Component performs arithmetic, logical and shift operations on data
• Arithmetic operations, Add, Subtract, Multiply and Divide
• Logical operations: AND, OR, NOT, XOR
• Shift operations: Move bits to the left or right within a register

Question 12

Accumulator

Answer 12

A general purpose register used to store the outputs of the ALU

Question 13

Give the different dedicated registers

Answer 13

• Program Counter
• Current Instruction Register
• Memory Address Register
• Memory Data Register
• Accumulator

Question 14

Fetch-execute cycle

Answer 14

Processors operate in defined stages that are used to cary out program instructions

Question 15

Program Counter

Answer 15

Holds the memory address of the next instruction to be executed

Question 16

Current Instruction Register

Answer 16

Holds the current instruction, which is split into opcode and operand

Question 17

Memory Address Register

Answer 17

Holds the address in memory where the processor is required to fetch or store data from or to

Question 18

Memory Data Register

Answer 18

Temporarily holds data moving between the processor and main memory

Question 19

Accumulator

Answer 19

To hold intermediate results of an instruction

Question 20

Fetch step 1

Answer 20

Address of next instruction transferred from PC to MAR

Question 21

Fetch Step 2

Answer 21

Instruction stored in location addressed by MAR is transferred to MDR

Question 22

Fetch Step 3

Answer 22

PC incremented by 1

Question 23

Fetch Step 4

Answer 23

Instruction transferred from MDR to CIR

Question 24

Decode step 5

Answer 24

Instruction held in the CIR is decoded

Question 25

Decode step 6

Answer 25

It is split into operand and opcode to determine the type of instruction it is. Additional data (if required) is fetched from memory

Question 26

Decode step 7

Answer 26

Gets passed to the accumulator
- The opcode specifies the operation that is to be carried out
- The operand holds either the address of the data to be used, which is then copied to the MAR or the actual data to be operated on which is passed to the MDR.