1.1.1 - Structure and Function of the Processor

Question 1

What is a register?

Answer 1

Register - Small piece of temporary memory located within the processor/CPU. Used for a specific purpose.

Question 2

What is the CIR and what is its purpose?

Answer 2

Current Instruction Register (CIR) - Holds the instruction while it is being decoded/executed.

• Contents are split into opcode and operand
• operand is copied to MAR if it is an address or MDR if it is data**

Question 3

What is the MDR and what is its purpose?

Answer 3

Memory Data Register (MDR)- Contains the instruction or data which has been accessed from memory

Question 4

What is the MAR and what is its purpose?

Answer 4

Memory Address Register (MAR) - Contents of the PC are copied here in order for the processor to access the next instruction

Question 5

What is the ACC and what is its purpose?

Answer 5

Accumulator (ACC) - Temporary storage for data being processed (input and output)

• Holds results of calculations (from the ALU)
• Checked for conditional branching (BRZ/BRP)
• Stores data which has come from the MDR/RAM

Question 6

What is the PC and what is its purpose?

Answer 6

Program Counter (PC) - Stores the address of the next instruction to be fetched

Question 7

What is the CU and what is its purpose?

Answer 7

Control Unit (CU) – Decodes instructions and sends out control signals

Question 8

What is the ALU and what is its purpose?

Answer 8

The Arithmetic and Logic Unit (ALU) - Carries out calculations and logical instructions

Question 9

What is the purpose of the address bus?

Answer 9

Address Bus – Carries the address where the data is going to or from

Question 10

What is the purpose of the data bus?

Answer 10

Data Bus - Carries the data between the CPU and memory.

Question 11

What is the purpose of the control bus?
What CPU component would use this bus?

Answer 11

Control Bus - Carries control signals from the control unit to other parts of the system

Question 12

What are the sub-stages of the fetch part of the FDE cycle?

Answer 12

1. Data/address is copied from PC to MAR
2. PC is incremented (usually by 1)
3. Data in MAR is passed onto the Address Bus
4. Control unit sends read signal onto the control bus
5. RAM copies the data from the location specified by the address bus along the data bus
6. Data on the data bus is stored in the MDR
7. Data is copied from the MDR to the CIR

Question 13

What are some key facts about the Von Neumann Processor

Answer 13

• Data and Instructions are stored in the same memory unit.
• Uses the same bus for data and instructions - can only fetch either data or instructions at one time
• Design is less complex (less transistors) - development is cheaper
• Single Control Unit / Arithmetic and Logic Unit
• Uses the special registers (MDR, ACC etc..)

Question 14

What are some key facts about the Harvard Processor

Answer 14

• Separate memory units for instructions and data - could fill unequally
• Has separate data and address buses for instructions
• Can read/write data and instructions simultaneously
• Has fixed memory sizes for data and instructions
• Development is more expensive. More CPU pins are required; a complex motherboard is required with doubling of memory
• Instruction memory may be ROM

Question 15

What is a contemporary architecture and what features may it have?

Answer 15

AKA - Modern Processors - Has aspects of both Von Neumann and Harvard.

Have features such as:

• Pipelining
• Makes use of cache
• Virtual Cores
• Hyper Threading
• Multiple Cores
• On-board Graphics

Question 16

What are the recognised factors affecting the performance of a CPU?

Answer 16

1. Clock Speed
2. Number of Cores
3. Cache Size/Speed

Question 17

What is meant by clock speed, and how does it influence the performance of the CPU?

Answer 17

• The speed at which the fetch decode execute cycle is completed.
• Higher clock speed gives more cycles per second meaning more instructions can be executed per second (program takes less time to run)

Question 18

How does the number of cores influence the performance of the CPU?

Answer 18

Each core is a processing unit, giving the potential for multiple instructions to be run at the same time

More instructions can be executed per second, so a program takes less time to run.

Question 19

How does the cache size influence the performance of the CPU?

Answer 19

High levels of cache give more space for data/instructions so slower RAM needs to be accessed less frequently.

Question 20

What is pipelining? What are the benefits?

Answer 20

• Makes the CPU more efficient
• Concurrent processing of multiple instructions – increases the number of instructions processed in a set period (increased speed of execution).
• Would allow one instruction to be fetched as the previous one is being decoded and the one before that is being executed.
• Prevents the CPU having to wait – No idle components reducing latency

Question 21

What issues does pipelining face?

Answer 21

Struggles with code that uses branching, as wrong instruction may be brought, meaning pipe flush needed in order to bring correct instruction.