The Purpose of the CPU

Question 1

What is the CPU?

Answer 1

Made up of billions of transistors (on-off switches). The CPU processes data, carries out instructions and control the computer’s components

Question 2

The Fetch-Decode-Execute cycle

Answer 2

• Fetches instructions from memory
• Decodes these instructions
• Then executes them

Question 3

Arithmetic Logic Unit (ALU)

Answer 3

Responsible for:
- Arithmetic operations (add, subtract etc)

• Logical operators (AND, OR etc)

Question 4

Control Unit (CU)

Answer 4

It co-ordinates the activity of the CPU by:

• Fetching and decoding instructions from memory
• Sending signals to control how data moves around the components and memory to execute the instructions

Question 5

Cache Memory

Answer 5

• Temporary storage that the CPU can access very quickly
• Used to store instructions that are frequently or likely to be used
• Faster than extracting the instructions from the memory

Question 6

Registers

Answer 6

• Memory locations within the CPU that temporarily hold data
• Can be accessed very quickly
• They accept, store, and transfer data and instructions for immediate CPU use
• The registers are used during the Fetch-Decode-Execute cycle

Question 7

Von Neumann architecture

Answer 7

• Data and instructions are both stored as binary digits
• Data and instructions are both in primary storage (memory)
• Instructions are fetched from memory one at a time and in order (serially)
• The CPU decodes and executes the instruction before moving onto the next

Question 8

What are the 4 registers in Von Neumann Architecture?

Answer 8

Program Counter (PC)
Memory Data register (MDR)
Memory Address Register (MAR)
Accumulator (ACC)

Question 9

Program Counter (PC)

Answer 9

• Keeps track of the memory location of the next instruction
• As each instruction gets fetched, the Program Counter increments by 1

Question 10

Memory Data Register (MDR)

Answer 10

• Stores any data fetched from memory
• Stores any data that is to be transferred and stored in memory

Question 11

Memory Address Register (MAR)

Answer 11

• Stores the address of the current instruction that is to be fetched from memory
• The memory address of where the data needs to go

Question 12

Accumulator (ACC)

Answer 12

• Stores the results of any calculations made by the ALU
• Stores the value of inputs and outputs to and from the CPU

Question 13

What happens in the FETCH stage?

Answer 13

1 The address of the next instruction is copied
from the PC and placed in the MAR

2 The MAR now holds a memory address. The CU
FETCHES the data at that address and copies it
to MDR

3 The PC is incremented by 1 to point to the next instruction

Question 14

What happens in the DECODE stage?

Answer 14

4 The MDR now contains the instruction. The CU then DECODES the instruction

Question 15

What happens in the EXECUTE stage?

Answer 15

5 The decoded instruction is EXECUTED. Maybe the ALU has to do a calculation