07 Fundamentals of Computer Architecture

Question 1

State the basic internal components of a computer system.

Answer 1

• CPU
• Input/output devices
• Storage

Question 2

What is the role of the processor?

Answer 2

The part of the computer which processes data by executing program instructions in the form of machine code which the processor is designed to handle based on a specific processor instruction set.

Question 3

Describe the role of the main memory.

Answer 3

Memory that can be directly accessed by the processor

Question 4

Outline the function of the address bus.

Answer 4

• Uni-directional bus
• carries the address of the memory location that data needs to be retrieved from or written to
• From processor to main memory and I/O controllers
• width of bus refers to number of bits that can be used to form the address of a memory location in main memory

Question 5

What is the function of the data bus?

Answer 5

• Bi-directional bus
• Carries data between the internal components of the system allowing values to be written or read
• width of bus refers to number of bits that can be transferred in one operation
• if width is the same as the computer word then retrieving data from a single memory address can be done in one operation

Question 6

Describe the function of the control bus.

Answer 6

• used to send control signals which manage and orchestrate the operations of the computer system
• these signals can include: clock signal, memory write/read and bus request

Question 7

Outline the role of the I/O controllers.

Answer 7

• The I/O controller acts as an interface between a peripheral device and the processor of the computer
• Peripherals cannot be directly connected to the processor as they work in different ways, speeds and with different signals

Question 8

Define device drivers.

Answer 8

Software which interacts directly with I/O controller that must be installed in order to communicate with a peripheral

Question 9

Briefly outline what a system bus is.

Answer 9

The system bus is a set of parallel connections that allow internal components to communicate with each other and exchange data.

Question 10

Define a word.

Answer 10

the amount of data in terms of bits a CPU’s internal data registers can hold and process at one time

Question 11

Define the stored program concept.

Answer 11

Machine code instructions are loaded into main memory to be executed by the processor.
These instructions are fetched one at a time and executed by the processor immediately in a sequential order.

Question 12

What are the features of von Neumann Architecture?

Answer 12

• Uses same address bus and data bus for instructions and data
• von Neumann architecture allows for the instructions and data to be saved in the same memory
• can be exploited by hackers who could disguise instructions (malware) as data that the processor may execute unknowingly when attempting to read the data
• Commonly used in general-purpose computers as it enables a more flexible use of the main memory, which allows the processor to run a variety of programs

Question 13

Describe the features of Harvard architecture.

Answer 13

• separate memory units and buses for instructions and data, which means that both memories can be accessed simultaneously (improves processor performance)
• each memory can be adapted to meet the needs of a particular system
• Used in embedded systems, the instruction memory can be implemented as a read-only memory (ROM), which protects the programs from accidental or deliberate changes by hacking
• Used in embedded systems where speed of operation is crucial and minimal storage is available e.g. washing machines, traffic lights

Question 14

Arithmetic logic unit (ALU)

Answer 14

A component within the processor which is responsible for performing arithmetic calculations and logical operations.

Question 15

Control unit

Answer 15

• responsible for directing the operations of all the other components of the processor
• uses and sends control signals to read data, decodes instructions by splitting them into opcode and operand, organised sequence of micro-operations to execute instruction

Question 16

Clock

Answer 16

• generates regular clock pulses by emitting a signal that continuously oscillates between a low and a high state
• The clock signal is used to synchronise the operations of the processor components.

Question 17

General-purpose registers

Answer 17

• temporarily store and access the results of operations
• keep the results of the intermediate calculations that are produced as part of a larger computation

Question 18

Program counter

Answer 18

Holds the address of the next instruction to be executed by the processor

Question 19

Memory Address register

Answer 19

Temporarily holds the address of the memory location that the processor needs to access, either to read from or write to.

Question 20

Memory Data/Buffer register

Answer 20

Temporarily holds the data that are read from or written to the main memory.

Question 21

Current instruction register

Answer 21

Holds the current instruction that the processor is executing

Question 22

Status register

Answer 22

• Used to store information about the result of the last instruction that the ALU executed.
• Each bit within the status register acts as a flag to indicate if an error or exception has occurred within the process

Question 23

Accumulator

Answer 23

Stores the result of any calculation processed by the ALU

Question 24

Describe the steps of the fetch-decode-execute cycle.

Answer 24

FETCH
1. The memory location address is transferred from the program counter to the MAR
2. The program counter is incremented by 1
3. Memory read signal sent across control bus to main memory
4. Data/instructions from memory location is placed on data bus and stored in MBR
5. Instruction transferred from MBR to CIR
DECODE
6. Instruction is decoded in control unit by splitting into opcode and operand
7. Additional data is fetched from memory and passed to registers
EXECUTE
8. Instruction is executed by the ALU
9. GP registers store intermediate values of calculations
10. The final result is stored in the accumulator

Question 25

Define processor instruction set.

Answer 25

the set of instructions that a particular CPU or computer architecture can execute

Question 26

What is the difference between direct and immediate addressing mode?

Answer 26

direct is where the operand holds the address of the datum as either a register or location in main memory

immediate is where the operand is the datum that needs to be processed

Question 27

Outline the opcode and operand of an instruction.

Answer 27

Opcode always includes the actual command the processor needs to execute e.g. ADD, MOV and the addressing mode

Operand contains one or more items of data (values, registers or address in memory) that are to be used in the instruction

Question 28

What is an interrupt?

Answer 28

• An interrupt is a signal that is sent to the processor to request immediate attention
• The processor suspends what it is doing and runs the process associated with the interrupt

e.g. I/O interrupt = if you started a transfer of a file from main memory to a USB stick and you removed the USB during transmission without warning

Question 29

What is the role of ISR?

Answer 29

Interrupt Service Routine (ISR)
a mini program that has been designed to respond to an interrupt’s request.
ISRs can be built into an operating system or provided via device drivers.

Question 30

What are the factors which affect performance of the processor and how?

Answer 30

multiple cores:

• more cores a computer has, the more instructions it can execute at the same time;
• but the system needs to spend time organising which cores receive which data and instructions

cache memory:

• larger the cache, the more instructions can be queued and carried out
• Storing instructions in cache reduces the amount of time it takes to access that instruction and pass it to a CPU core

clock speed

• he greater the clock speed, the faster instructions are carried out
• overclocking could cause the processor to overheat.

word length

• The larger the word size, the greater the amount of data that can be transferred to the CPU in one pass.
• to process an instruction it might well require multiple fetches of data from the main memory if word length is too short

address bus width

• The greater the width of the address bus, the more memory locations can be addressed
• the processor benefits from a larger main memory to access data and instructions
• reduces reliance on slower virtual memory

data bus width

• The larger the data bus, the better the processor performance. - the more data can be transferred between the internal components simultaneously

Question 31

Outline what cache memory is.

Answer 31

a fast, relatively small capacity set of memory locations that sit close to the processor
it is used to store the instructions and data most frequently used.

Question 32

Outline the principles of operation of barcode readers.

Answer 32

1. To read the code, a light from a laser illuminates the barcode. More light is reflected from the white areas than from the black bars.
2. The reflected light is captured by one or more photoelectric cells that generate a set of electrical pulses that correspond to the black and white stripes in the barcode.
3. These pulses are processed and converted to a binary number that represents the code.

Question 33

Outline the principles of operation of digital cameras.

Answer 33

1. When the shutter is opened, light is allowed through and passes through the lens
2. The lens focuses the light onto an image sensor (an array of millions of photosites) so the image is broken up into millions of pixels
3. Photosites only measure light intensity so a Bayer filter (a series of red, blue and green filters that sit above the image sensor) is used to determine the colour of the pixel
4. The final value of the pixel is determined from its colour intensity and those of neighbouring pixels through a demosaicing algorithm
5. The image goes through a analogue to digital converter so each pixel is represented by a numerical code

Question 34

Outline the principles of operation of laser printers.

Answer 34

1. The data to be printed is sent to the printer driver
2. Printer driver puts data into a format so it can be printed
3. Printer driver checks status of printer e.g. is it out of toner?
4. The data is sent to the printer and stored in the printer buffer
5. The printer drum is coated in negative charge
6. The printer drum rotates and a laser beam is passed over it to remove the charge in certain areas leaving positive areas corresponding to the text or images you are printing
7. The printer drum is coated in negatively charged toner which only sticks to the positive areas
8. Positively charged paper is passed over the drum
9. The toner sticks to the paper making an exact copy of the text/images printed
10. Electrical charge is then removed to stop the page from sticking to the drum
11. The page is passed through a fuser to bond the ink to the page making a permanent copy
12. A discharge lamp is used to remove any charge on the drum ready for the next page

Question 35

Outline the principles of operation of RFID.

Answer 35

RFID (radio frequency identification) allows data to be transmitted wirelessly over radio waves.

RFIDs use a transponder and receiver
1. Powered receiver emits radio frequency energy
2. The transponder antenna in a mobile phone, credit card or tag is energised by the reciever
3. The transponder can now send data to the reciever

Question 36

What are the uses of RFIDs?

Answer 36

• Identification of people, goods, animals or valuables
• Shipping and supply chain tracking for goods
• Banking and fast-payment systems

Question 37

What is the difference between passive and active RFIDs?

Answer 37

Passive transponders such as those in bank cards themselves have no power - they rely on the radio waves emitted from the reciever.

Active tags have a larger, battery-powered beacon which can broadcast its own signal for receivers up to 300 m away

Question 38

What is the need for secondary storage in a computer system?

Answer 38

• We need non-volatile storage
• secondary storage needs to be high capacity and low cost
• It needs to be fast enough for loading and saving files
• needs to provide reliable permanent storage so that files are not lost or easily corrupted

Question 39

Outline the principles of operation of a hard disk.

Answer 39

• A hard disk is made of many metal platters
• Binary data is stored on these platters as a series of polarised states
• The platter can be broken up into a series of concentric tracks and these can be further broken up into sectors
• The platters spin at high speeds and the positive and negative charges are read by the read/write head and converted into binary
• The read/write head polarises particles on the platter to write binary data

Question 40

What are the characteristics of hard disks?

Answer 40

Capacity: Large (up to 4 TB)
Cost: Cheap per GB of storage e.g. 1 TB = £50
Durability: lots of moving parts so easily damaged BUT its less likely to degrade or corrupt over long periods of disuse
Portability: it is portable
Speed: medium speeds - affected by disk latency (seek delay head moving and rotational delay disk turning under head)

Question 41

What are the principles of operation for optical discs?

Answer 41

• A high powered laser burns pits onto the surface of the optical disk to write data as pits and lands
• A low powered laser is used to read the data - a light sensor detects any reflections (pits deflect the light so it isn’t detected and recorded as a 1)

Question 42

What are the characteristics of optical discs?

Answer 42

Capacity: Medium (Blu-Rays up to 50 GB)
Cost: low costs
Durability: susceptible to scratching, easy to break and not always reliable
Portability: easily portable
Speed: slow read write speeds

Question 43

What are the principles of operation of SSDs?

Answer 43

• consists of NAND flash memory AND a controller that manages pages, and blocks and complexities of writing
• floating gate transistors trap and store charge
• a block is made up of many pages

Question 44

What are characteristics of SSDs?

Answer 44

Capacity: large capacity
Cost: very expensive
Durability: resistant against mechanical failure, susceptible to memory cell ‘leakage’, limited lifespan as number of times it can be written to is finite.
Portability: better than HDD due to no moving parts
Speed: very high speeds of retrieval and writing