1.1.1 Structure & Function of the Processor

Question 1

What are all the registers in a CPU?

Answer 1

Program Counter (PC)
Accumulator (ACC)
Memory Address Register (MAR)
Memory Data Register (MDR)
Current Instruction Register (CIR)

Question 2

What does the PC do?

Answer 2

Holds the address of the next instruction to be executed.

Question 3

What does the ACC do?

Answer 3

Stores results of calculations.

Question 4

What does the MAR do?

Answer 4

Holds the address of the location that is to be read from or written to.

Question 5

What does the MDR do?

Answer 5

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

Question 6

What does the CIR do?

Answer 6

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

Question 7

What does the Control Unit (CU) do?

Answer 7

Directs the operations of the CPU by sending control signals to areas of the CPU to coordinate the operations.

Question 8

What does the Arithmetic & Logic Unit (ALU) do?

Answer 8

Arithmetic operations (e.g. +, -, /, *) and logical operations (e.g. AND, OR, NOT, XOR)

Question 9

What does the data bus do?

Answer 9

Transports data and instructions between components. Bi-directional.

Question 10

What does the address bus do?

Answer 10

Transmits the memory addresses that specify where data is to be sent or retrieved from. Uni-directional.

Question 11

What does the control bus do?

Answer 11

Transmits control signals between internal and external components. Bi-directional.

Question 12

What does the opcode and operand contain?

Answer 12

Opcode => The type of instruction to be executed

Operand => The data or address of the data upon which the operation is to be performed.

Question 13

What is pipelining and how does it work?

Answer 13

Increases the efficiency of the CPU by reducing the amount of time it’s kept idle.

It does this by completing the FDE cycles of 3 separate instructions simultaneously.

Question 14

What happens in the FDE cycle?

Answer 14

Fetch => The address of the next instruction is copied from the PC to the MAR. The instruction held at this address is copied from the MAR to the MDR via the data bus. Simultaneously, the PC is incremented by 1. The value held in the MDR is copied to the CIR.

Decode => The contents of the CIR are split into opcode and operand.

Execute => The instruction is executed and the ALU performs any necessary calculations. The result is stored in the ACC.

Question 15

Why does doubling the amount of cores not necessarily double the speed of the CPU?

Answer 15

Not all programs are designed to make use of multiple cores efficiently.

Question 16

What are the main features of Von Neumann architecture? What are its advantages?

Answer 16

Shared memory
Shared bus for data and instructions
Single CU and ALU
Special registers within the CPU.

Advantages => Processors are cheaper to develop as the CU is easier to design, programs can be optimised in size.

Question 17

What are the main features of Harvard architecture? What are its advantages?

Answer 17

Physically separate memories for data and instructions.
Used for embedded systems.

Advantages => Quicker execution as data & instructions are fetched in parallel, more space efficient as memories can be different sizes.

Question 18

What are the main features of Contemporary architecture? What are its advantages.

Answer 18

Combines Von Neumann & Harvard. Von Neumann used for main memory, Harvard used for the cache.

Advantages => Uses cache, has multiple cores, uses pipelining, 2 separate areas for memory which increases performance.