SLR 01 Structure & Function Of The Processor

Question 1

What Is The ALU?

Answer 1

• Arithmetic Logic Unit
• Deals with basic arithmetic such as addition or subtraction
• Is capable of many complex mathematical functions such as sine, cosine and floating point division

Question 2

What Is The CU?

Answer 2

• Control Unit
• Co-ordinates the various units inside the processor via the use of “control signals” which determine the flow of data into, and around the processor.
• Instructions are decoded in the control unit during the FDE cycle.

Question 3

What Are Registers?

Answer 3

• A location where a single item of data can be stored.
• Specialized, high speed storage within the CPU itself.
• Before any data is processed, it must be represented in individual registers.

Question 4

What Registers Are Located In The Processor? (5)

Answer 4

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

Question 5

What Does The Program Counter (PC) Do?

Answer 5

Stores the address of the next instruction

Question 6

What Does The Memory Address Register (MAR) Do?

Answer 6

Contains the address of the memory location currently in use.

Question 7

What Does The Memory Data Register (MDR) Do?

Answer 7

Contains the instructions/data to be executed.

Question 8

What Does The Current Instruction Register (CIR) Do?

Answer 8

Holds the instruction whilst it is being decoded.

Question 9

What Does The Accumulator (ACC) Do?

Answer 9

The accumulator stores the results of calculates processed by the ALU.

Question 10

Why Are The CIR and MDR Different Registers If They Seem To Store The Same Value?

Answer 10

It may seem as though the CIR and MDR always hold the same value. However, consider an input instruction, if there was no CIR, the MDR would hold the input instruction. However when the data is input, the inputted value would overwrite the input instruction in the MDR, hence the need for the CIR and the MDR.

Question 11

What Are Buses?

Answer 11

Buses are a collection of wires through which data is transmitted from one part of a computer to another.

Question 12

What Are The Three Data Buses?

Answer 12

Data Bus
Address Bus
Control Bus

Question 13

What Does The Data Bus Do?

Answer 13

• Transfers Data
• Typically From CPU Ram
• Half duplex, data can travel in either direction.

Question 14

What Does The Address Bus Do?

Answer 14

• Transfers addresses of memory being saved to / loaded from memory
• Simplex pathway, the CPU only ever sends the memory address.

Question 15

What Does The Control Bus Do?

Answer 15

• Sends and receives signals from all parts of the computer.
• Ensures that processes occur at the correct time.
• These signals travel along the control bus
• Made of several control lines including an interrupt request line, a memory read line and a memory write line, in addition to the clock for synchronizing operations.

Question 16

What Is The FDE Cycle?

Answer 16

The fetch, decode and execute cycle describes the standard steps needed for processing to take place.

Question 17

What Steps Are Taken During The Fetch Part Of The Cycle?

Answer 17

1. The Address in the Program Counter is copied to the Memory Address Register.
2. The Program Counter is incremented.
3. A Fetch signal is sent along the Control Bus by the Control Unit to the Memory, while the address held in the Memory Address Register is sent to the Memory along the Address Bus. The value stored in memory returns to the CPU along the Data Bus and is held in the Memory Data Register.
4. This value in the Memory Data Register is copied to the Current Instruction Register.

Question 18

What Steps Are Taken During The Decode Part Of The Cycle?

Answer 18

1. The contents of the Current Instruction Register are sent to the Control Unit for decoding.
2. The type of addressing used by the instruction must be identified:
   - If the address is a direct address, load this address into the Memory Address Register and retrieve the contents of that address.
   - If the address is an indexed address, add the contents of the index register to the address, then copy this value to the Memory Address Register. Retrieve the contents of this memory location.

Question 19

What Steps Are Taken During The Execute Part Of The Cycle?

Answer 19

1. If the instruction is a jump:
   1. Load the address operand into the Program Counter and copy the address component in the Current Instruction Register to the Program Counter.
   2. At this point execution has finished.
2. Otherwise, execute the instructions.

Question 20

Which Factors Affect CPU Performance?

Answer 20

• Clock Speed
• Cache Memory
• Number Of Cores
• Pipelining

Question 21

What Is Clock Speed?

Answer 21

• The number of FDE cycles the CPU can perform per second.

Question 22

What Is Cache Memory?

Answer 22

• Built into the CPU
• Much quicker to access than RAM
• Very expensive and therefore small

Question 23

What Are Multiple Cores?

Answer 23

• Each core can operate independently of the others

- Each have their own low level cache.