Unit 1.1.1: Structure and Function of the Processor

Question 1

What is the CPU, and what does it manage?

Answer 1

The ‘Central Processing Unit’ processes data and instructions by means of calculating, sorting and searching.

The CPU has a number of different components, each with their own role to perform:

. Control Unit
. Buses
. Arithmetic-Logic Unit (ALU)
. Dedicated registers

Question 2

What is the ‘Control Unit’, and how does it perform its purpose?

Answer 2

It is the component of the processor that coordinates the activity of all other components.

Control signals are sent along the control bus between the control unit and the other components of the computer.

Question 3

What is a bus, and name and explain the different types of bus?

Answer 3

Buses in a computer comprise of a series of connectors that transfer signals between internal components.

There are three different types of bus that carry control signals, addresses and data, which are held altogether within a system bus:

. Address bus: the width of this bus determines the maximum possible memory address of the system (i.e. an 8-bit address bus has the maximum memory address of 2^8 = 256).

. Data bus: it is bi-directional, as well as the width of this bus being defined by the number of wires or lines it contains. If the data bus is the same width as a computer ‘word’, data can be transferred in a single operation.

. Control bus: it is unidirectional, as it carries control signals from the processor to other components of the computer. It also carries the system clock’s pulses.

Question 4

What do the control signals comprise of?

Answer 4

. Memory read: causes data from the addressed location in RAM to be placed on the data bus.

. Memory write: causes data on the data bus to be written into the addressed location in RAM.

. Bus request: indicates that a device is requesting use of the data bus

. Bus grant: indicates that the CPU has granted access to the data bus.

. Clock: used to synchronise operations.

Question 5

What is the ‘Arithmetic-Logic Unit’ (ALU)?

Answer 5

It is the problem solving part of the processor, which comprises of performing arithmetic, logical and shift operations on data.

Question 6

What is the ‘accumulator’?

Answer 6

A general purpose register that passes the results from the ALU to be stored temporarily in several memory locations called registers.

Question 7

Results passed from the ALU to the registers. Why is this beneficial to the processor?

Answer 7

The processor would be able to immediately access and reuse these results in subsequent calculations.

Question 8

State the dedicated registers of the CPU.

Answer 8

. Program counter (PC)

. Current Instruction Register (CIR)

. Memory Address Register (MAR)

. Memory Data Register (MDR)

. Accumulator

Question 9

Describe the ‘Fetch’ of the FDE cycle.

Answer 9

1. The address of the next instruction is copied from the PC to the MAR.
2. The instruction held at that address is copied to the MDR, relaying the instruction via the main memory.
3. Simultaneously, the contents of the PC are incremented.
4. The contents of the MDR are copied to the CIR.

Question 10

Describe the ‘Decode’ of the FDE cycle.

Answer 10

1. The instruction held in the CIR is decoded by becoming split into ‘operand’ and ‘opcode’ to determine the type of instruction it is. Additional data, if required, is fetched from memory and passed to the accumulator.
2. The ‘opcode’ specifies the operation that is to be carried out.
3. The ‘operand’ holds either:
   - The address of the data to be used, to be copied to the MAR; or
   - The actual data to be operated on, to be passed onto the MDR.

Question 11

Describe the ‘Execute’ of the FDE cycle.

Answer 11

The actual data, after being decoded by the CIR, becomes executed and the result is held in the accumulator, or stored in the main memory.