Topic 1: Components of a computer

Question 1

What are the components of the Central Processing Unit?

Answer 1

1. Control Unit
2. Busses (Address bus, Control bus, Data bus)
3. Arithmetic / Logic Unit (ALU)
4. Registers (Accumulator, Program Counter, Current Instruction Register, Memory Address Register, Memory Data Register)

Question 2

What is the purpose of the Control Unit?

Answer 2

The control unit controls and coordinates the activities of the CPU.

Question 3

What is the job of the Address Bus?

Answer 3

The Address Bus carries the address of the piece of memory or input/ output devices to be read from or written to

Question 4

What is the job of the Data Bus?

Answer 4

The Data Bus consists of 8, 16, 32 or 64 separate lines which provides a two-way path for data and instructions moving between system components

Question 5

What is the job of the Control Bus?

Answer 5

The Control Bus is a two-way path. The purpose of the control bus is to transmit command, timing and specific status information between system components

Question 6

What is the purpose of the ALU (Arithmetic and Logic Circuit)

Answer 6

The purpose of the ALU is to perform arithmetic and logical operations of the data, it can perform instructions such as ADD, SUBTRACT, MULTIPLY, DIVIDE on fixed or floating point numbers.

Question 7

What is the purpose of the Program Counter?

Answer 7

The program counter holds the address of the next instruction to be executed.

Question 8

What is the purpose of the Current Instruction Register?

Answer 8

The current instruction register holds the current instruction being executed, divided into operand and opcode

Question 9

What is the purpose of the Memory Address Register?

Answer 9

The Memory Address register holds the address of the memory location from which data (or an instruction) is to be fetched or to which data is to be written

Question 10

What is the purpose of the Memory Data Register?

Answer 10

The Memory Data Register stores the data being transferred to and from storage, it contains a copy of the designated memory locations specified by the Memory Address Register.

Question 11

What is the purpose of the Accumulator?

Answer 11

The Accumulator stores the resultant of each calculation from the ALU

Question 12

Explain each step of the Fetch part of the Fetch-Decode-Execute cycle.

Answer 12

1. The address of the next instruction is copied from the program counter (PC) to the memory address register (MAR)
2. The instruction held at that address is copied to the memory data register (MDR). Simultaneously, the content of the program counter (PC) is incremented so that it holds the address of the next instruction.
3. the contents of the memory data register (MDR) are copied to the current instruction register (CIR)

Question 13

Explain the Decode part of the Fetch-Decode-Execute cycle.

Answer 13

4. The instruction held at the current instruction register (CIR) is decoded. The instruction is split into opcode and operand, the opcode is used to determine the type of instruction and what hardware to use to execute it.

Question 14

What will the operand hold?

Answer 14

The actual data to be operated on, which will be copied to the MDR

The operand will hold either the:
#The address of the data to be used with the operation, which is then copied to the MAR

Question 15

Explain the Execute part of the Fetch-Decode-Execute cycle.

Answer 15

5. the appropriate instructions / opcode is carried out on the operand.

Question 16

What are the control lines for the control bus?

Answer 16

Bus Request- Indicates that a device is requesting the use of the data bus
Bus Grant- indicates that the CPU has granted access to the data bus
Memory Write- causes the data on the data bus to be written into the addressed location
Memory Read- causes the data from the addressed location to be placed on the data bus
Interrupt Request- Indicates that a device is requesting access to the CPU
Clock- Used to synchronise operations