1.1.1 Architecture of the CPU

Question 1

What is the CPU?

Answer 1

The processor or Central Processing Unit (CPU) is the internal hardware component of the computer that is responsible for executing the instructions of programs

Question 2

what is the CPU made up of?

Answer 2

• arithmetic logic unit (ALU)
• control unit (CU)
• registers

Question 3

what is the ALU?

Answer 3

The Arithmetic and Logic Unit (ALU) is the component of the CPU that carries out the following:

• Arithmetic calculations: addition, subtraction, division, and multiplication
• Logical operations: such as AND, OR, NOT
• Comparisons between values, such as greater than, less than, equal to

Question 4

what is the CU?

Answer 4

The control unit organises the execution of instructions, including managing the other components in the processor.

The control unit is responsible for the following tasks:

• Ensures the execution of instructions in the correct sequence
• Decodes every instruction that the processor will execute
• Sends and receives control signals to and from other components
• Checks that signals have been delivered successfully
• Makes sure that data goes to the correct place at the correct time

The control unit also contains the clock. This is a tiny component that controls the rate at which instructions are executed in the processor.

Question 5

what is the clock

Answer 5

The clock signal is used to synchronise the operations of the processor components. The system clock (also simply referred to as the clock) generates regular clock pulses by emitting a signal that continuously switches between a low (or ‘0’) and a high (or ‘1’) state.

The time taken between two sequential rising edges is called a clock cycle or a clock period. The clock speed is measured by the number of clock cycles in one second — 1 clock cycle per second is 1Hz.

Herz Measurement
1 hertz (Hz) 1 clock cycle
per second
1 million Hz megahertz (MHz)
1,000 MHz gigahertz (GHz)

Question 6

what are registers?

Answer 6

Registers are locations of computer memory within the processor that provide extremely fast access.

General-purpose registers are used to store the results of the intermediate calculations that are part of larger computations.

Dedicated or special-purpose registers are processor-based registers that are used for a specific purpose.

• CIR
• MAR
• MDR
• PC
• Accumulator

Question 7

What does the current instruction register (CIR) do?

Answer 7

Holds the current instruction that the processor is executing

Question 8

what does the memory address register (MAR) do?

Answer 8

Temporarily stores the memory addresses used when searching for data in RAM

Question 9

what does the memory data register (MDR) do?

Answer 9

Temporarily holds the data (data values or instructions) that are read from or written to the main memory

Question 10

what does the program counter (PC) do?

Answer 10

Holds the address of the next instruction to be executed by the processor

Question 11

what does the accumulator do?

Answer 11

Stores the result of any calculation

Question 12

what is the fetch decode execute cycle?

Answer 12

The fetch-decode-execute cycle describes the basic operation of modern computers.

Instructions are loaded into memory (RAM) before the processor starts running the program

Question 13

what is the fetch stage of the FDE cycle?

Answer 13

Each instruction is fetched from memory (in order), and put into the appropriate registers. The control unit can then access the instruction for the next stages

Question 14

what is the decode stage of the FDE cycle?

Answer 14

The instructions (which are received in binary) need to be decoded before they can be run. This is the process the control unit uses to work out what the other components need to do.

Each processor will have slightly different encodings for instructions

Question 15

what is the execute stage of the FDE cycle?

Answer 15

The control unit will tell the other components what they need to do in order for the instruction to work

Question 16

what is computer architecture?

Answer 16

Computer architecture refers to the structure and organisation of a computer system. It specifies the components that make up a computer system and describes how these are interconnected, how they interact with each other, and how they are managed

Question 17

what is Von Neumann architecture?

Answer 17

In the 1940s, John von Neumann and his team developed the concept of the stored program computer. The Von Neumann architecture used the idea of storing program instructions and data in main memory and moving them between memory and the processor. Von Neumann architecture is used in many modern-day computer systems.

The von Neumann architecture consists of:
- a Processor
- a Memory unit that can communicate directly with the processor
- connections for input and output devices
- Secondary storage for saving/backing up data

The processor can access the instructions and data in the main memory as required to execute the program. It does this by using dedicated connections called buses:

• an address bus is used to identify the addressed location
• a data bus is used to transfer the contents to/from that location

This means that the same address and data buses are used in the process of transferring instructions and data between main memory and the processor. A third bus, the control bus, is used to synchronise and control operations.