Unit 3.1 - computer architecture

Question 1

What is the CPU?

Answer 1

The central processing unit/microprocessor is an integrated circuit on a single microchip made out of billions of transistors (tiny electrical switches) that combine to form logic gates. It fetches, decodes and executes instructions.

It processes data/instructions, performs arithmetic calculations and logical operations. It controls operations and processing of a computer. Responsible for the Fetch-Decode-Execute cycle.

Does not directly carry out input, output or storage functions.

Question 2

What is the CU?

Answer 2

The control unit coordinates all the activities happening in the CPU and manages the flow of data around the CPU/between the CPU and other devices. Manages the other components (e.g. ALU). Initiates each step of the FDE cycle and is responsible for decoding the instruction in the CIR.

Question 3

What is the PC?

Answer 3

The program counter is a register that holds the address of the next instruction to be fetched.

Increments by 1 when instruction is fetched.

Question 4

What is MAR?

Answer 4

Memory address register is a register that contains the current address of the RAM location that data should be written to or read from.

Question 5

What is the MDR?

Answer 5

Memory data register is a register used to temporarily store data that is read from/written to RAM.
All data going to/from memory must pass through here.

Question 6

What type of value does a microprocessor use to help it make a decision?

Answer 6

Stored value (compares sensor data to a value stored in memory)

Question 7

List all the components of the CPU:

Answer 7

1. Control unit (CU)
2. Arithmetic Logic unit (ALU)
3. Floating point unit (FPU)
4. Cache memory (IAS)
5. Program counter (PC)
6. Current instruction register (CIR)
7. Accumulator (ACC)
8. Memory address register (MAR)
9. Memory data register (MDR)

Question 8

What is the ALU?

Answer 8

Arithmetic logic unit.
Carries out arithmetic calculations and logical operations (e.g. bit shifts)

Uses the Accumulator to temporarily store data while performing these calculations.

Only works with integer values.

Question 9

What is the FPU?

Answer 9

Floating point unit.

Performs arithmetic calculations on non-integer values.

Question 10

What are registers?

Answer 10

Small, temporary areas of storage within the CPU.

Extremely fast to access because they are located directly within the CPU.

Question 11

What is the CIR?

Answer 11

The current instruction register stores the instruction which is currently being decoded/executed after it was fetched from memory.

Question 12

What is the ACC?

Answer 12

The accumulator is a register used to temporarily hold the data being worked on by the ALU/FPU or the result of the final calculation.

Question 13

What is a bus?

Answer 13

An electronic pathway that components of the CPU use to communicate with each other and RAM.

Question 14

State the 3 types of buses

Answer 14

1. Control bus - carries control signals and command codes. From CU to other components or from CPU to RAM. Usually unidirectional
2. Address bus - carries addresses from MAR and RAM to indicate which address will be used to read from/write data into. Unidirectional.
3. Data bus - carries data and instructions. E.g. carries data written from MDR to RAM or vice versa. Bidirectional.

Question 15

Explain an example of the value 50 being written into location ‘10’ in RAM

Answer 15

• The value 50 is carried from the MDR to the RAM via a data bus
• The value 10 is carried from the MAR to the RAM via an address bus
• A write signal is sent from the CPU to the RAM via a control bus

Question 16

What is the fetch-decode-execute cycle?

Answer 16

Also known as the stored program concept or the Von Neumann model, it is a cycle that computers follow to run programs.
Fetch - instruction is fetched from RAM to CPU
Decode - instruction is sent from CIR to be decoded by CU, determines what type of instruction it is
Execute - instruction is executed, like an arithmetic calculation or storing result/data in RAM. Usually performed by ALU. Often includes storing results.

Question 17

Breakdown of fetch stage

Answer 17

1. The PC contains the address of the next instruction. Instructions are fetched 1 at a time.
2. The contents of the PC are copied to the MAR
3. The PC is incremented by 1, so that it still points to the next instruction
4. Address bus carries the value from MAR to RAM
5. The control bus sends a read signal to RAM
6. Data bus carries contents of desired RAM address from RAM to MDR
7. Contents of MDR are copied to CIR, ready to be decoded

Question 18

What 4 things affect CPU speed?

Answer 18

1. Clock speed - clock refers to a pulse/signal used to synchronize processes occurring within the computer. Clock speed is measured in mega/gigahertz and refers to the number of ticks/second. Determines how many cycles a CPU can perform each second. Higher clock speed = more heat and energy generated/used.
2. Cores - a processor within a CPU. 2 cores allows a CPU to work on to instructions simultaneously. Modern CPUs have 2-16 cores. Having multiple cores doesn’t always mean better performance, as some software only work with one core, while the others remain idle.
3. Instruction sets - different processors have different machine code instructions, some more efficient than others, therefore making some processors faster.
4. Cache memory - small amounts of very fast temporary/volatile memory located in the CPU. Used to store frequently accessed data to allow faster access to it without having to go to RAM. Greater cache size = more data it can store = fewer requests to RAM = faster CPU performance.

Question 19

What does the speed of the CPU affect?

Answer 19

The speed at which the CPU can perform FDE cycles affects its performance.
More cycles completed each second = program running faster

Question 20

What is an instruction set?

Answer 20

The list of instructions that a CPU can process. When written in binary, it is known as machine code.

Question 21

What is an embedded system?

Answer 21

A specialized computer system/single microprocessor that performs a single, dedicated function inside another device/appliance (e.g. security system).
Also known as dedicated system.
Limited functionaility but have some advantage over general computers such as : cheaper, increased reliability, simple interface.

Question 22

What does the CPU access first when it tries to read data from a memory location?

Answer 22

It will first check if it exists in cache memory. If it does, it will access the data from there. If the data does not exist there, it will be accessed from the comparatively slower RAM.