4.7 - Fundamentals of computer organisation and architecture

Question 1

What is main memory?

Answer 1

A place where the computer stores program instructions and frequently used data, consisting of the RAM (random access memory) and ROM (read only memory).

Question 2

What is the benefit of main memory?

Answer 2

It is much faster than secondary storage so storing frequently used data helps the processor execute instructions quickly.

Question 3

What is a bus?

Answer 3

A series of parallel wires that connects internal components of a computer, allowing signals to pass between them.

Question 4

What is the address bus?

Answer 4

A bus used to transport memory addresses, specifying where data is to be sent or retrieved from.

Question 5

What is the data bus?

Answer 5

A bus used to send data and instructions to and from different components of a computer.

Question 6

What is the control bus?

Answer 6

A bus used to carry control signals that regulate the operation of the computer system. It also carries the clock signal.

Question 7

What are I/O controllers?

Answer 7

Pieces of hardware that control the communication of data between the processor and external hardware devices.

Question 8

How do Harvard and Von Neumann architecture differ?

Answer 8

Harvard : Two separate memory locations (and sets of buses), one for instructions, the other for data.

Von Neumann : Both instructions and data stored in same memory and share the same buses

Question 9

Why is Harvard architecture considered better than Von Neumann architecture?

Answer 9

It avoids the bottleneck of a single set of buses and avoids the possibility of data being executed as instructions. It also allows each part of memory to have different characteristics, such as making instruction memory read only.

Question 10

What is the stored program concept?

Answer 10

A computer that uses the stored program concept serially fetches and executes machine code instructions stored in main memory by a processor that performs arithmetic and logical operations.

Question 11

Why is the stored program concept important?

Answer 11

It forms the foundation of modern computers. Early computers were designed to execute one specific program, but the stored program concept allows one set of instructions to be switched out for another, so numerous different applications can be run.

Question 12

What is a processor’s instruction set?

Answer 12

The group of instructions that it can carry out. Each type of processor has its own instruction set, so instructions for one may not be compatible with others.

Question 13

What are the two primary parts of an instruction?

Answer 13

Opcode, which specifies the type of operation as well as the addressing mode.

One or more operands, which are the pieces of data on which the operation is performed.

Question 14

What is immediate addressing?

Answer 14

The value specified in the operand is to be treated as the actual value to use.

Question 15

What is direct addressing?

Answer 15

The value specified in the operand signifies a memory address to access.

Question 16

What is an interrupt?

Answer 16

A signal sent to the processor by another part of the computer requesting the attention of the processor.

Question 17

How are interrupts handled?

Answer 17

It is first detected as a change in the content of the status register between the execute and fetch stages of the FDE cycle.

Then, the processor stops executing the current program and places the content of its registers onto the system stack, known as ‘saving the volatile environment’.

Now that it has saved its progress, loads appropriate interrupt service routine (series of instructions for handling a specific interrupt).

Once ISR finished, processor restores volatile environment from system stack and resumes execution of any programs running before the interrupt.

Question 18

How does the number of cores affect processor performance?

Answer 18

Each core can perform its own FDE cycle independently of others, so different applications can be allocated different cores. This means that multiple tasks can be processed at the same time, increasing efficiency.

Question 19

How does cache memory affect processor performance?

Answer 19

Cache memory is a small portion of incredibly fast memory, with read and write speeds far higher than HDDs and even SSDs. It stores frequently used information and reduces time wasted in fetching the same information from main memory time and time again. The more cache memory, the more information it can store and the more time it can save in fetching information from main memory.

Question 20

How does clock speed affect processor performance?

Answer 20

The higher the frequency of pulses, the more cycles of the FDE cycle can be completed per unit time. However, errors can occur when clock speed is increased too far, and the processor can overheat.

Question 21

How does word length affect processor performance?

Answer 21

A word is a group of bits treated as a single unit by the processor, and can represent instructions or data. Word length is the number of bits assigned to it, so higher word length allow more bits to be transferred and manipulated as a single unit.

Question 22

How does address bus width affect processor performance?

Answer 22

Increasing width increases range of addresses it can specify, hence increasing computer’s amount of addressable memory.

Question 23

How does data bus width affect processor performance?

Answer 23

Increasing width increases volume of data that can be transferred over the bus at any one time. More data can be fetched per cycle, reducing number of cycles required to fetch large volumes of data.

Question 24

How does a barcode reader work?

Answer 24

Consists of a laser light source, a lens, photodiodes and a mirror.

Mirror directs laser onto barcode. Light reflected by barcode passes through lens and is incident on photodiode which turns light into electrical charge. Charge measured and processed to form digital signal representing content of barcode.

Barcodes have error detection and prevention built in such as parity bits and check digits, which indicate if barcode has been read correctly.

Question 25

How does a digital camera work?

Answer 25

Consists of a lens that focuses light onto a sensor, and a shutter that regulates the path of light between lens and sensor.

Sensor converts incident light into electrical charge. Charge builds up in cells, each of which represents a pixel. Once photo taken, charge in each cell measured and converted to digital value, which is processed and stored as digital image.

In colour cameras, multiple cells for each pixel, each with filter. Lets camera build up separate image for intensity of each colour, which is combined to form full colour image.

Question 26

How does a laser printer work?

Answer 26

Consists of laser light source, mirror, drum, toner roller and fusers.

Drum positively charged all over before laser directed at its surface by mirror. Areas where laser is incident are discharged, leaving impression of page in electrical charge on drum. Toner roll dispenses negatively charged toner onto drum, which is attracted to positively charged portions of drum. Toner applied to paper by drum before paper is heated by fusers, fixing toner to paper.

Colour printers use four different colours of toner.

Question 27

How does RFID work?

Answer 27

Inside RFID tag is a chip containing small amount of memory. Chip attached to coil of wire which acts as an antenna.

When tag scanned, reader emits radio waves which are picked up by tag’s antenna. Power induced in antenna is enough to power chip, which emits own radio wave through antenna, containing info held on chip. Wave picked up by reader, decodes it and returns info to computer.

Most tags are passive, induce power wirelessly from reader. Active tags contain small power supply to power chip, and can be used much further away.

Question 28

How does a hard disk drive work?

Answer 28

Consists of a number of circular platters made from magnetic material. Above each plate hovers actuating arm, on which is a read/write head, which changes magnetic polarity of parts of the platter.

Data written in concentric tracks, each divided into sectors. Platter rotates 1000s of times per minute. Binary data stored on platter as a series of polarised states representing either 1s or 0s.

Capacity 500GB - 5TB. Adding more platters and decreasing width of tracks increases capacity.

Susceptible to damage from movement, so unsuitable for portable devices.

Question 29

How does a solid-state drive work?

Answer 29

Consists of NAND flash memory cells and a controller that manages the structure of data on the drive. NAND flash memory is non-volatile, meaning contents retained even with no power.

Memory cells formed from floating gate transistors which store info by trapping electrical charge.

Data stored in pages, which are combined to form blocks.

SSDs not capable of overwriting data - must completely erase entire page before writing new info.

No moving parts so higher read/write speeds than HDD, and suitable for portable devices.

Question 30

How does an optical disk work?

Answer 30

Info read optically by laser. Either read-only, recordable or rewritable. Consists of series of pits and lands. Pits are burned into surface by high-power laser, permanently deforming surface. One continuous track which spirals from centre. When low-power laser passes over lands it reflects back onto photodiode. However, when incident on pit, light is scattered and not reflected back. Pattern of reflections and scatters converted to digital binary signal.

Recordable and rewritable: Opaque dye instead of pits.

Recordable: Photosensitive dye changes from opaque to transparent under high-powered laser which is used to write info to disk. Dye unaffected by low-power read laser.

Question 31

Storage capacity of HDD vs SSD vs Optical disk

Answer 31

HDD: High, 500GB - 5TB, typically 1TB
SSD: Relatively low, typically 256GB
Disk: Very low - 700MB (CD) - 25GB (Blue-ray)

Question 32

Speed of HDD vs SSD vs Optical disk

Answer 32

HDD: Good, 100MB/s
SSD: Very high, 500MB/s
Disk: Relatively low, 30MB/s