1.1 Systems Architecture

Question 1

The purpose of the CPU

Answer 1

The CPU is the Central Processing Unit.

It executes instructions stored in memory by repeatedly carrying out the fetch-decode-execute cycle.

Question 2

Summarise the Fetch-Decode-Execute Cycle

Answer 2

The FDE cycle or Fetch-Execute cycle describes the basic operation of modern computers.
Instructions are loaded into main memory (RAM) before the processor starts running the program.
The instructions are FETCHED from memory.
Once fetched, the instructions are DECODED by the Control Unit within the CPU, to decide what to do.
Then the instructions are EXECUTED.
Every operation is carried out within the FDE cycle is regulated by the cycle of the CPU clock.

Question 3

Summarise the Fetch phase of the FDE Cycle

Answer 3

1. Each instruction is FETCHED from main memory (RAM) in order.
2. Instructions are put in to the appropriate registers.
3. The Control Unit (CU) accesses instructions for the next stages.

Question 4

Summarise the Decode phase of the FDE Cycle.

Answer 4

Instructions (in binary) need to be decoded before they can be run.
The Control Unit (CU) DECODES them to work out what the other components need to do.

Question 5

Summarise the Execute phase of the FDE Cycle.

Answer 5

The Control Unit (CU) tells the other components what to do in order to carry out the instructions.
The cycle repeats.

Question 6

Common CPU Components

Answer 6

ALU = Arithmetic Logic Unit
CU = Control Unit
Cache
Registers

Question 7

What is the ALU and its function?

Answer 7

ALU = Arithmetic Logic Unit
Carries out:
1. arithmetic calculations: + - * / (binary shifts)
2. logical operations: AND OR NOT
3. comparisons: < > ==

Question 8

What is the CU and its function?

Answer 8

CU = Control Unit
Simple: Co-ordinates all of the CPU’s actions in the FDE Cycle.
Medium: Organises the execution of instructions, including managing the other components in the CPU.

Detailed:

1. Ensures instructions executed in correct order
2. Decode every instruction the processor will execute
3. Send and receive signals to and from other components
4. Check that signals have been delivered successfully
5. Make sure data goes to the correct place at the correct time.

Question 9

Describe the function of Cache

Answer 9

Cache is a small amount of high-speed RAM built directly within the processor.
It is used to temporarily hold data and instructions that the processor is likely to reuse.
This allows for faster processing as the processor does not have to wait for the data and instructions to be fetched from RAM.

Question 10

Describe the function of registers.

Answer 10

Registers are even smaller and faster than cache.
Memory locations within the CPU.
Temporarily store memory addresses, instructions or data
32 bits / 64 bits.

Question 11

Von Neumann Architecture

Answer 11

John von Neumann developed the stored program computer.
In a Von Neumann computer, both programs and data are stored in memory.
Von Neumann architecture consists of:
1. a processor
2. a memory unit that can communicate directly with the processor
3. connections for input and output devices
4. secondary storage for saving/backing up files

Question 12

What is the function of MAR?

Answer 12

MAR = Memory Address Register
Found within the Control Unit (CU)
Temporarily holds the address (location in memory) of the current instruction or piece of data to be fetched/stored.

Question 13

What is the function of MDR?

Answer 13

MDR = Memory Data Register
Found in the ALU.
Temporarily holds the data or program instruction when it is fetched from memory
or holds data that is waiting to be written to memory.

Question 14

What is the function of the Program Counter?

Answer 14

PC = Program Counter
Found in the Control Unit (CU)
Register which holds the memory address of the next instruction to be processed.

Question 15

What is the function of the Accumulator?

Answer 15

Found in the ALU.

Register in which the results of operations carried out in the ALU are stored.