Structure and Function of the Processor

Question 1

What is inside the CPU (Central Processing Unit )

Answer 1

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

Question 2

What does the Arithmetic Logic Unit (ALU) do

Answer 2

Completes all the arithmetical and logical operations

Question 3

Name the different Control Units and their function

Answer 3

• PC (Program Counter) - holds the address of the next instruction to be executed by the processor
• ACC (Accumulator) - stores the results from calculations
• MAR (Memory Address Register) - temporarily holds the address of the memory location that the processor needs to access
• MDR (Memory Data Register) - temporarily holds the data that the processor needs to access
• CIR (Current Instruction Register) - holds the current instruction that the processor is executing divided into opcode and operand

Question 4

What are the functions of the Control Unit

Answer 4

• Directing the operations inside of the CPU
• Decodes instructions
• Sends control signals to coordinate movement of data through the processor / execute instruction
• Controls buses

Question 5

Describe two ways in which the accumulator is used

Answer 5

• Temporary storage during calculations.
• I/O in processor and is used as a buffer.

Question 6

What are registers

Answer 6

Small memory cells inside of the CPU that operate at high speeds and temporarily store data

Question 7

What is a bus

Answer 7

A set of parallel wires which connect two or more components inside the CPU

Question 8

What is the width of a bus

Answer 8

The number of parallel wires it has, this is directly proportional to the number of bits that can be transferred simultaneously at any given time

Question 9

What are the three buses inside the CPU

Answer 9

• Address Bus
• Data Bus
• Control Bus

Question 10

Describe the function of the three buses

Answer 10

• Address Bus - transmits the memory addresses between components
• Data Bus - bi-directional bus that transmits data and instructions between components
• Control Bus - bi-directional bus that transmit control signals between components

Question 11

What is the system bus

Answer 11

The collection of the data bus, address bus and control bus

Question 12

What is the width of the address bus proportional to

Answer 12

The number of addressable memory locations

Question 13

What does adding a wire to the address bus do to the number of addressable locations

Answer 13

It doubles the number of addressable locations

Question 14

Describe the different control signals and their functions

Answer 14

• Bus request - shows that a device is requesting the use of the data bus
• Bus grant - shows that the CPU has granted access to the data bus
• Memory write - data is written into the addressed location using this bus
• Memory read - data is read from a specific location to be placed onto the data bus
• Interrupt request - shows that a device is requesting access to the CPU
• Clock - used to synchronise operations

Question 15

What is Assembly Language

Answer 15

• A low level language that uses mneomincs to represent instructions
• The instruction is divided up into operand and opcode

Question 16

What is Opcode

Answer 16

Specifies the instruction to be performed and the memory location

Question 17

What is Operand

Answer 17

Holds a value related to the data on which the instruction is to be performed

Question 18

What is the main function of a CPU

Answer 18

Fetch, Decode, Execute

Question 19

Describe what happens during a Fetch

Answer 19

• Contents of PC are copied into MAR
• MAR is sent along the address bus to RAM
• The Control unit instructs a memory read operation to allow the contents of the memory location to be transferred
• The data now stored in RAM is placed and sent along the data bus
• The Data Bus loads the Instructions into MDR
• The contents of the MDR are copied into the CIR
• Finally, the control units increments the PC by 1

Question 20

Describe what happens during a Decode

Answer 20

The contents of the CIR is split into operand and opcode

Question 21

Describe what happens during a Execute

Answer 21

The opcode is executed on the data

Question 22

What are the three factors that affect the performance of the CPU

Answer 22

• Clock Speed
• Number of Cores
• Size of Cache

Question 23

How does the clock speed function

Answer 23

• Clock speed is determined by the system clock
• All processor activities begin on a clock pulse
• Each CPU operation starts as the clock changed from 0 to 1

Question 24

What is a core within a computer

Answer 24

• A core is an independent processor that is able to execute its own fetch-execute cycle
• A computer with multiple cores can complete more than one fetch-execute cycle at any given time
• However, some programs aren’t optimised for the use of more than one core

Question 25

What is cache memory

Answer 25

* The CPU’s onboard memory which can be acessed a lot faster than main memory * Instructions fetched from main memory are copied to the cache, so if required again it can be accessed quicker * As cache fills up, unused instructions get replaced

Question 26

Describe the 3 Levels of Cache

Answer 26

* Level 1 Cache - Very fast memory cells with a small capacity * Level 2 Cache - Relatively fast memory cell, with a medium sized capacity * Level 3 Cache - Much larger and slower memory cell

Question 27

What is Pipelining

Answer 27

* When Processes are arranged in a series, the output of one process is the input of next * The process of completing the fetch, decode, and execute cycles of three separate instructions simultaneously which increases the speed of execution

Question 28

Give two examples of where pipelining is used in any computer system

Answer 28

* Instruction processing - some processors allow parts of instructions to be processed without waiting to complete the whole instruction cycle * GPUs - contain lots of seperate processors that can render parts of an image at the same time through pipelining

Question 29

How does pipelining improve CPU performanace

Answer 29

* Reduces CPU idle time * While one instruction is being executed, another can be decoded and another fetched * This means instructions are exeuted at a faster rate

Question 30

What two forms is pipelining broken down into

Answer 30

* Instruction pipelining - separating out the instruction into fetching, decoding, and executing * Arithmetic pipelining - breaking down the arithmetic operations and overlapping them as they are performed

Question 31

What are the potential hazards of pipelining

Answer 31

* Two instructions could be trying to use address bus and data bus at the same time * An instruction cannot continue because it needs a value that has not yet been generated by an earlier instruction

Question 32

What is Von Neuman Architecture

Answer 32

Architecture in which there is a single shared memory and 1 set of buses meaning there is a shared data bus for both data and instructions

Question 33

Describe some of the features of Von Neumann Architecture

Answer 33

* Single control unit * One instruction at a time * Uses fetch execute cycle * Shared memory for data and instruction * Single set of shared buses

Question 34

What is Harvard Architecture

Answer 34

Architecture in which there are two separate memories and data buses for data and instructions, this is more commonly used with embedded processors

Question 35

What is Contemporary Processing

Answer 35

* A combination of Harvard and Von Neumann architecture * Von Neumann is used when working with data and instructions in main memory, but uses Harvard architecture to divide the cache into instruction cache and data cache * Makes use of cache, core and pileing

Question 36

What are the advantages and disadvantages of Von Neuman Architecture and Harvard Architecture

Answer 36

Von Neumann Architecture - * Advantages - Cheaper to develop as the control unit is easier to design. Programs can be optimised in size * Disadvantages - The CPU is much faster than the data bus, meaning it often sits idle. Some programs can overwrite another in memory, causing it to crash Harvard Architecture * Advantages - Quicker execution as data and instructions can be fetched in parallel. Memories can be different sizes, which can make more efficient use of space * Disadvantages - Limited flexibility as there is only a certain amount of data that can be used for data and certain amount for instructions