1.1.1 - CPU, ALU, Registers, Buses, FDE Cycle

Question 1

CPU - What is it?

Answer 1

The Central Processing Unit is a processor that has a number of components which enable it to execute instructions.

Question 2

CPU - What are it’s components?

Answer 2

• Control Unit
• Buses
• ALU (Arithmetic Logic Unit)
• Registers

Question 3

Control Unit - What is it and What does it do?

Answer 3

A component on the CPU that controls and coordinates activity of the CPU. Directs the flow of data between the CPU and other devices.

Question 4

Control Unit - Process

Answer 4

• Accepts next instructions
• Decodes it into separate steps
• Manages it’s execution
• Stores the result in memory or registers

Question 5

Buses - What are they?

Answer 5

A set of parallel wires connecting two or more components of a computer. They are essentially paths that allow data to flow across.

Question 6

Buses - Process

Answer 6

The processor is connected to the main memory by three separate buses. When the CPU wants to access a main memory location it sends the address to the address bus. The data in that location is returned to the CPU on the data bus. Control signals are sent along the control bus.

Question 7

Buses - Control Bus | What is it used for?

Answer 7

Used to transmit a command, timing and status info between components. Data can flow along the control bus both ways. Control lines are provided to stop conflict.

Question 8

Buses - Data Bus | What is it used for?

Answer 8

Used for moving data and instructions between components. Data can flow both ways along the data bus.

Question 9

Buses - Address Bus | What is it used for?

Answer 9

Used to store memory addresses from which data will be fetched or sent and stored

Question 10

Buses - Address Bus | What are words and what do they do?

Answer 10

Memory is divided into units called words. A word is a fixed size group of digits. Each word in memory has it’s own address and the address bus transmits the memory addresses of words that are used as operands in program instructions.

Question 11

ALU - What does it do?

Answer 11

The arithmetic logic unit performs logical operations on the data such as ADD, SUBTRACT, MULTIPLE or DIVIDE. It can also perform shift operations which is shifting bits to the left or right within a register and can carry out Boolean logic operations comparing two vales using operations such as AND, OR, NOT

Question 12

Registers - What are they?

Answer 12

Special memory cells that operate at high speed

Question 13

Registers - What are stored in them?

Answer 13

Results of all arithmetic, logical or shift operations are temporarily stored in registers

Question 14

Registers - Accumulator | What is it and what does it store?

Answer 14

A simple register that stores all the results from operations.

Question 15

Registers - What are some examples of special purpose registers?

Answer 15

• Program Counter (PC)
• Current Instruction Register (CIR)
• Memory Address Register (MAR)
• Memory Data Register (MDR)

Question 16

Registers - Program Counter | What does it do?

Answer 16

Holds the address of the next instruction to be executed

Question 17

Registers - CIR | What does it do?

Answer 17

Holds the current instruction being executed in the format of operand and opcode

Question 18

Registers - MAR | What does it do?

Answer 18

Holds the address of the memory location from which data is to be fetched or written

Question 19

Registers - MDR | What does it do?

Answer 19

Temporarily holds the data read from or written to memory. AKA Memory Buffer Register

Question 20

Fetch Decode Execute Cycle - What is it?

Answer 20

A sequence of operations involved in executing an instruction

Question 21

FDE Cycle - What are the steps of the fetch phase?

Answer 21

1. The address of the next instruction is copied from the PC to the MAR
2. The instructions held at that address is copied to the MDR and the PC is incremented by 1 and gets a new address
3. The contents of the MDR are copied to the CIR

Question 22

FDE Cycle - What are the steps of the decode phase?

Answer 22

4. The instruction held in the CIR is decoded then the instruction is split into opcode and operand and the opcode is used to determine the type of instruction and what hardware is used to execute it.

Question 23

FDE Cycle - What are the steps of the execute phase?

Answer 23

5. The appropriate instruction/opcode is carried out on the operand

Question 24

Opcode - What is it?

Answer 24

Operation code is an instruction that can be executed by the CPU

Question 25

Operand - What is it?

Answer 25

The operand specifies the location/data that needs to be acted on