3.1 Computer Architecture

Question 1

What are the 5 registers used in the Von Neumann architecture?

Answer 1

• Program counter (PC)
• Memory address register (MAR)
• Memory data register (MDR)
• Current instruction register (CIR)
• Accumulator (ACC)

Question 2

What is the purpose of the ALU?

Answer 2

Arithmetic Logic Unit

• Carries out calculations on data e.g.
  
  • arithmetic operations: add, subtract …
  • logical operations: AND, OR …
• Holds temporary values used during calculations …
  
  • .. in a register called the accumulator (ACC)

Question 3

What is the purpose of the Control Unit (CU)?

Answer 3

Control Unit

• It contains the Current Instruction Register (CIR) and the Program Counter (PC).
• Manages the flow of data and interaction between the components of the processor
• Signals are generated along the control bus to tell the components of the computer what to do
• Reads/fetches instructions from memory

Question 4

What is the IAS?

Answer 4

Immediate Access Store

• Used to hold data and instructions before they are processed e.g. registers, RAM, cache

Question 5

Purpose of the Program Counter (PC)?

Answer 5

• Contains the address of the next instruction to be fetched
• Increments to point to the address of the next instruction to be fetched

Question 6

Purpose of the Memory Address Register (MAR)

Answer 6

• Holds the address(location) of the current instruction/data to be fetched
• The contents of the PC are copied here and …
  
  • … then transferred along the Address Bus

Question 7

Purpose of the Memory Data Register (MDR)?

Answer 7

• Temporary storage between the Central Processing (CPU) and primary memory
• Contains the instructions/data after it is fetched from main memory

Question 8

Purpose of the Current Instruction Register (CIR)?

Answer 8

• Contains the instructions to be decoded
• It is copied from the MDR

Question 9

Purpose of the accumalator (ACC)?

Answer 9

Holds data (temporarily) that is currently being used in calculations by the Logic Unit

Question 10

What are the 3 buses used in the Von Neumann architecture?

Answer 10

• Address bus
• Control bus
• Data bus

Question 11

What is the purpose of the 3 buses used in the Von Neumann architecture?

Answer 11

• Buses are used to connect together the internal components of the CPU

Question 12

Purpose of the address bus?

Answer 12

• Transports the memory address …
  
  • … between processor and memory
  • … of the next item to be fetched
• Data travels one way (unidirectional) CPU -> Memory

Question 13

Purpose of the control bus?

Answer 13

• Carries (transports) signals …
  
  • … that control the actions of the CPU
  • … that synchronise the fetch-execute cycle
• Data can travel one way (unidirectional) or both ways (bidirectional)

Question 14

Purpose of the data bus?

Answer 14

• Carries (transports) data …
  
  • … that is currently being processed
  • … between processor, memory and peripherals(input/output devices)
• Data travels both ways (bidirectional)

Question 15

Explain how an instruction is fetched in a computer based on the Von Neumann model

Answer 15

• PC contains the address of the next instruction to be fetched
• The address held in the PC is copied to the MAR … and placed on the address bus
• The instruction is transferred to the MDR from the memory location contained in the MAR … using the data bus
• The instruction in the MDR is copied to the CIR
• The PC is incremented by 1
• The instruction is then decoded and executed by the Control Unit

Question 16

Describe the stored program concept when applied to the Von Neumann model?

Answer 16

• Program stored on a secondary storage device
• Data & instructions are moved to RAM/Memory
• Data & Instructions are stored in the same memory
• Data & instructions are moved to registers to be executed
• Instructions are fetched one at a time and executed one after another

Question 17

To process instructions the CPU goes through 3 main stages. What are they?

Answer 17

• Fetch
• Decode
• Execute

Question 18

What is being described?

• Carries out calculations on data e.g.
  
  • arithmetic operations: add, subtract …
  • logical operations: AND, OR …
• Holds temporary values used during calculations …
  
  • .. in a register called the accumulator (ACC)

Answer 18

Arithmetic Logic Unit

Question 19

What is being described?

• It contains the Current Instruction Register (CIR) and the Program Counter (PC).
• Manages the flow of data and interaction between the components of the processor
• Signals are generated along the control bus to tell the components of the computer what to d
• Reads/fetches instructions from memory

Answer 19

Control Unit (CU)

Question 20

What is being described?

• Used to hold data and instructions before they are processed e.g. registers, RAM, cache

Answer 20

Immediate Access Store (IAS)

Question 21

What is being described?

• Contains the address of the next instruction to be fetched
• Increments to point to the address of the next instruction to be fetched

Answer 21

Program Counter (PC)

Question 22

What is being described?

• Holds the address(location) of the current instruction/data to be fetched
• The contents of the PC are copied here and …
  
  • … then transferred along the Address Bus

Answer 22

Memory Address Register (MAR)

Question 23

What is being described?

• Temporary storage between the Central Processing (CPU) and primary memory
• Contains the instructions or data after it is fetched from main memory

Answer 23

Memory Data Register (MDR)

Question 24

What is being described?

• Contains the instructions to be decoded
• It is copied from the MDR

Answer 24

Current Instruction Register (CIR)

Question 25

What is being described? * Holds data (temporarily) that is currently being used in calculations by the Logic Unit

Answer 25

Accumalator (ACC)

Question 26

What is being described? * Transports the memory address ... * * ... between processor and memory * ... of the next item to be fetched * Data travels one way (unidirectional)

Answer 26

Address Bus

Question 27

What is being described? * Carries (transports) signals ... * … that control the actions of the CPU * ... that synchronise the fetch-execute cycle * Data can travel one way (unidirectional) or both ways (bidirectional)

Answer 27

Control bus

Question 28

What is being described? * Carries (transports) data … * … that is currently being processed * … between processor, memory and peripherals(input/output devices) * Data travels both ways (bidirectional)

Answer 28

Data bus

Question 29

What is being described? * PC contains the address of the next instruction to be fetched * The address held in the PC is copied to the MAR ... and placed on the address bus * The instruction is transferred to the MDR from the memory location contained in the MAR ... using the data bus * The instruction in the MDR is copied to the CIR * The PC is incremented by 1 * The instruction is then decoded and executed by the Control Unit

Answer 29

Fetching an instruction in the Von Neumann model

Question 30

The CPU consists of …

Answer 30

* a Control Unit (CU) * an Arithmetic and Logic Unit (ALU) * registers * buses

Question 31

What is responsible for the execution of instructions and the processing of data

Answer 31

CPU

Question 32

What 3 factors affect the performance of a CPU?

Answer 32

* Number of cores * Cache size * Clock speed

Question 33

Instruction sets are different for each different CPU architecture. Each instruction is made up of …

Answer 33

An **Opcode** and an **Operand**

Question 34

Each processor instruction is made up of a an opcode and an operand. What does the operand represent?

Answer 34

The operand is the data or memory address

Question 35

Each processor instruction is made up of a an opcode and an operand. What is the purpose of the opcode?

Answer 35

The opcode informs the CPU what operation needs to be done.

Question 36

CPU - what is meant by clock speed?

Answer 36

* It determines the number of fetch-decode-execute cycles per second * The higher the clock speed, the faster a CPU can execute instructions

Question 37

CPU - what is meant by overclocking?

Answer 37

* A CPU will often have a maximum clock speed recommended by the manufacturer * You can increase the speed above this in the BIOS, which is known as overclocking

Question 38

Overclocking can improve the performance of a CPU, but what problem can it cause?

Answer 38

It can lead to overheating

Question 39

CPU - what is meant by number of cores?

Answer 39

* The number of processors linked on a single chip * Each core contains an ALU, CU and registers * A quad core processor contains 4 cores

Question 40

How does the number of cores affect CPU performance?

Answer 40

* Each core can process instructions simultaneously increasing performance, but …. * Doubling the number of cores doesn't necessarily double performance because * Cores need to communicate with each other and programs need to be written to use multiple cores

Question 41

CPU - What is meant by cache size?

Answer 41

* The amount of cache memory located within the CPU * Cache memory stores frequently used instructions and data

Question 42

How does the cache size affect CPU performance?

Answer 42

Cache memory read/write speeds are much faster than RAM The larger the cache size, the better the performance

Question 43

What is an embedded system?

Answer 43

* A combination of hardware and software which is designed to carry out a specific set of functions or tasks * They are feedback oriented systems that will often contain * a user interface * analogue or digital inputs * some form of output e.g. security systems

Question 44

What 3 things can an embedded system be based on?

Answer 44

* **microcontrollers** - They have a CPU in addition to some RAM and ROM and other peripherals all embedded onto one single chip (together they carry out a specific task) * **microprocessors** - An integrated circuit which only has a CPU on the chip (there is no RAM, ROM or peripherals - these need to be added) * **system on chips** - An integrated circuit that combines many elements of a computer system into a single chip. * This may contain a microcontroller as one of its components (they almost always will include CPU, memory, input/output [I/O] ports and secondary storage on a single microchip)

Question 45

What are the possible benefits of an embedded system?

Answer 45

* they are small in size and therefore easy to fit into devices * compared to other systems, they are relatively low cost to make * they can be controlled remotely using a mobile phone, for example * they are usually dedicated to one task allowing simple interfaces and often no requirement for an operating system * they consume very little power * very fast reaction to changing input (operate in real time and are feedback orientated) * with mass production comes reliability

Question 46

What are the possible drawbacks of an embedded system?

Answer 46

* it can be difficult to upgrade some devices to take advantage of new technology * troubleshooting faults in the device becomes a specialist task * although the interface can appear to be more simple (e.g. a single knob) in reality it can be more confusing (e.g. changing the time on a cooker clock can require several steps!) * any device that can be accessed over the internet is also open to hackers. viruses, etc. * due to the difficulty in upgrading and fault finding, devices are often just thrown away rather than being repaired (very wasteful) * can lead to an increase in the 'throw away' society if devices are discarded just because they have become out-of-date