1. Processor Components

Question 1

What are the components of the CPU?

Answer 1

1. Control Unit (CU)
2. Buses
3. Arithmetic Logic Unit (ALU)
4. Dedicated registers

Question 2

What are the roles of the Control Unit?

Answer 2

1. Controls and co-ordinates the activities of the CPU
2. Directs flow of data between CPU and other devices
3. Accepts next instruction
4. Decodes instruction into several sequential steps
5. Manages the execution
6. Stores resulting data in memory or registers

Question 3

What is a bus?

Answer 3

A set of parallel wires connecting two or more components of a computer

Question 4

Name the 3 buses.

Answer 4

1. Address bus
2. Data bus
3. Control bus

Question 5

How is the processor connected to the main memory?

Answer 5

Via the 3 buses

Question 6

What happens when the CPU wishes to access a particular main memory location?

Answer 6

1. Sends address to memory on address bus
2. Data in address location returned to CPU on data bus
3. Control signals sent along control bus

Question 7

What are the 3 buses called collectively?

Answer 7

System bus

Question 8

What is the role of the address bus?

Answer 8

To transmit memory addresses specifying where data is to be sent to or retrieved from

Question 9

What is the role of the data bus?

Answer 9

To transport data and instructions between components

Question 10

What are the roles of the control bus?

Answer 10

To transmit control signals between internal and external components
Co-ordinates use of address and data buses
Provides status information between system components

Question 11

Is the address bus bi-directional?

Answer 11

No

Question 12

Is the data bus bi-directional?

Answer 12

Yes

Question 13

Is the control bus bi-directional?

Answer 13

Yes

Question 14

Name the 6 control lines.

Answer 14

1. Bus Request
2. Bus Grant
3. Memory Write
4. Memory Read
5. Interrupt Request
6. Clock

Question 15

What does the Bus Request control line do?

Answer 15

Indicates that a device is requesting the use of the data bus

Question 16

What does the Bus Grant control line do?

Answer 16

Indicates that the CPU has granted access to the data bus

Question 17

What does the Memory Write control line do?

Answer 17

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

Question 18

What does the Memory Read control line do?

Answer 18

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

Question 19

What does the Interrupt Request control line do?

Answer 19

Indicates that a device is requesting access to the CPU

Question 20

What does the Clock control line do?

Answer 20

Synchronises operations

Question 21

How is memory divided internally?

Answer 21

Into units called words

Question 22

What is a word?

Answer 22

A fixed size group of digits

Question 23

What is the ALU used for?

Answer 23

Performing arithmetic (+, -, x, /) and logical (e.g. AND, OR, NOT, XOR, binary shifts) operations on data

Question 24

What are registers?

Answer 24

Small memory cells that operate at very high speeds

Question 25

What are registers used for?

Answer 25

Temporarily storing data | Site of arithmetic, logical and shift operations

Question 26

Name the 5 registers.

Answer 26

1. Program Counter (PC) 2. Accumulator (ACC) 3. Memory Address Register (MAR) 4. Memory Data Register (MDR) 5. Current Instruction Register (CIR)

Question 27

What is the purpose of the PC register?

Answer 27

Holds address of next instruction to be executed

Question 28

What is the purpose of the ACC?

Answer 28

Stores results from calculations

Question 29

What is the purpose of the MAR?

Answer 29

Holds address of the location to be read from or written to

Question 30

What is the purpose of the MDR?

Answer 30

Temporarily stores data that has been read or needs to be written

Question 31

What is the purpose of the CIR?

Answer 31

Holds the current instruction being executed, divided into operand and opcode

Question 32

What does the operand contain?

Answer 32

The data or address of the data which the operation is to be performed on

Question 33

What does the opcode contain?

Answer 33

The type of operation that needs to be executed

Question 34

What is the Fetch-Decode-Execute cycle?

Answer 34

Sequence of operations completed to execute an instruction

Question 35

What happens during the Fetch phase?

Answer 35

1. Address from PC is copied to MAR 2. Instruction held at that address is copied to MDR via data bus 3. Simultaneously, contents of PC incremented 4. Value held in MDR is copied to CIR

Question 36

What happens during the Decode phase?

Answer 36

1. Contents of CIR divided into operand and opcode 2. Operand may contain the following: a. address of data to be used (copied to MAR) b. actual data to be operated on (copied to MDR) c. data to be operated on may be passed to ALU or ACC

Question 37

What happens during the Execute phase?

Answer 37

1. Decoded instruction is executed