4.7 - Fundamentals of computer organisation and architecture

Question 1

What are the 6 basic internal components of a computer system?

Answer 1

Processor

Main memory

Address bus

Data bus

Control bus

I/O controllers

Question 2

What is the processor and what does it do? (2 points)

Answer 2

Sometimes referred to as the CPU - central processing unit

Processes data by executing program instructions - machine code instructions based on a specific instruction set

Question 3

How can a program be executed? (2 points)

Answer 3

Instructions need to be transferred from secondary storage into main memory from where they can be fetched, decoded and executed

The data that needs to be processed is also loaded into main memory or provided by input/output devices via I/O controllers

Question 4

What are the 5 main qualities of main memory?

Answer 4

Two types of main memory:
RAM - working memory used by processor during the fetch-decode-execute cycle
ROM - used in boot process for computer system

Instructions or data are stored as binary sequences in memory locations

Each one has a physical address - a number used to locate the location and access its contents

Distinct from secondary storage - can be accessed directly by the processor; secondary storage must be accessed through I/O controllers

Question 5

What are I/O controllers and what do they do? (6 points)

Answer 5

Components each device has that act as an interface between core computer system and peripherals

They connect all external and secondary storage devices to the processor

Not favourable for peripherals to be directly attached to processor - each device operates with different speeds and electronic signals

They provide the mechanism for:
- Input data to be received for processing from input devices like keyboards
- Computation results to be outputted from the system to output devices like display screens

Provide a set of addressable registers that the processor can access to communicate with I/O devices

Question 6

How do I/O controllers connect to devices? (3 points)

Answer 6

Physical port on the controller provides connection for the peripheral

Device has a cable/connector to connect it to the port

Large desktop computers usually have slots where additional specialist controllers can be added

Question 7

How do I/O controllers work? (4 points)

Answer 7

Controller circuitry facilitates exchange of data in the form of electrical signals between the processor and peripheral

Controller then translates signals into correct form

Peripheral device almost certainly operates slower than processor

Interface provides buffer - disparate timing requirements can be satisfied

Question 8

Define ‘device driver’. (3 points)

Answer 8

A piece of software that provides an interface for the peripheral device

Enables operating systems to access functions of hardware without having to be specifically configured for every device used

Many devices are now ‘plug and play’ - driver is either embedded or automatically downloaded when the hardware is connected

Question 9

What is a bus and what is the system bus? (2 points)

Answer 9

Bus - a communication system that is used to transfer data between components

System bus - a set of parallel connectionsthat allow internal components to communicate with each other and exchange data

Question 10

What are the data bus and address bus? (3 points)

Answer 10

Data bus - a bidirectional bus that transfers data and instructions

Address bus - a unidirectional bus that specifies the address of a memory location to read/write data

Width (number of parallel lines) = number of bits that can be used to form an address of a memory location

Question 11

What are 4 of the control bus’s functions?

Answer 11

Sends control signals to manage and orchestrate operations that take place inside a computer system

Exchangingstatus signals between the components of the computer system

Transmittingclock signals required for the coordination of operations

Specifying the type of data that is being transferred via the other buses (i.e. data values, instructions, or addresses)

Question 12

What are the 4 components of Von Neumann architecture?

Answer 12

A processor

A memory unit that can communicate with the processor directly

Connections for input and output devices

Secondary storage for saving/backing up data

Question 13

How does Von Neumann’s architecture work and where is it used? (4 points)

Answer 13

Uses the idea of storing program instructions and data in main memory and moving them between memory and the processor

Processor can access the instructions and data in the main memory as required to execute a program using buses

Same address and data buses used in transferringinstructions and databetween main memory and the processor

Commonly used in general-purpose computers that are expected to accommodate the varying needs of the end-users

Question 14

How does Harvard architecture work and where does it work? (3 points)

Answer 14

Keeps instructions and data in separate memories

Processor accesses the ‘instructions memory’ and ‘data memory’ using separate data and address buses

Used extensively in embedded systems (eg. DSP systems) and also in microcontroller devices and some types of smartphones

Question 15

What is the advantage of Von Neumann architecture?

Answer 15

Enables more flexible use of main memory - allows processor to run a variety of programs that are not known in advance

Question 16

What is the disadvantage of Von Neumann architecture?

Answer 16

Can be exploited by hackers who could disguise malware instructions as data that the processor may unknowingly execute

Question 17

What are the 2 advantages of Harvard architecture?

Answer 17

Allowing both memories to be accessed simultaneously minimises issue of keeping processor waiting while loading/saving data into memory - increases processor performance

Each memory can be adapted to meet the needs of a particular system - different sizes, word lengths or implemented using different technologies

E.g. the instruction memory can be implemented as ROM in systems with predetermined uses - protects programs from accidental or deliberate changes by hacking

Question 18

What is the stored program concept? (2 points)

Answer 18

Machine code instructions are stored in main memory

They are fetched and executed serially by a processor that performs arithmetic and logical operations

Question 19

What does the ALU (Arithmetic and Logic Unit) do? (5 points)

Answer 19

Responsible for performing arithmetic calculations and logical operations that include:
- Addition, subtraction, multiplication, division
- Logical bitwise operations, such as AND, OR, NOT, and XOR
- Comparisons between values, such as greater than, less than, equal to
- Shifting binary patterns to the left or right

Question 20

What does the CU (Control Unit) do? (5 points)

Answer 20

Responsible for directing the operations of all the other components of the processor

Uses control signals to:
- Enable data to be read (loaded) from and written (stored) to the main memory
- Determine the operation the ALU will carry out at each instance

Decodes every instruction (analyses an opcode and operand) that the processor will execute to determine what needs to be done

Organises the sequence of micro-operations that need to be performed for an instruction to be carried out

Question 21

What does the clock do? (5 points)

Answer 21

Used to synchronise the operations of the processor components

Generates regular clock pulses by emitting a signal that continuously oscillates between a low (0) and high (1) state

Rising/falling edge - a change from a low/high to a high/low state

Clock period - the time taken between two sequential rising edges (one clock cycle)

Every operation of the processor typically requires a number of clock cycles to complete - specific to each processor

Question 22

What are (general purpose) registers and what are their 3 functions?

Answer 22

Locations of computer memory within the processor that provide extremely fast access

Functions:
- Used to temporarily store and access the results of operations
- Prevents having to access main memory and instruction execution slowing down
- Used to keep results of intermediate calculations produced as a part of a larger computation (connected to ALU)

Question 23

What are dedicated registers?

Answer 23

Processor-based registers that are used for a specific purpose

Question 24

What are the 5 dedicated registers and their functions?

Answer 24

Program Counter (PC) - holds the address of the next instruction to be executed by the processor

Current Instruction Register (CIR) - holds the current instruction that the processor is executing

Memory Address Register (MAR) - temporarily holds the address of the memory location (in main memory) that the processor needs to access, either to read from or write to

Memory Buffer Register (MBR) - temporarily holds the data (data values or instructions) that are read from or written to the main memory

Status Register - stores information about the result of the last instruction that the ALU executed

Question 25

What are the 10 steps of the execution of an interrupt?

Answer 25

1. Processor receives the interrupt
2. Processor completes the fetch-decode-execute cycle of the instruction that it was running when interrupt was received
3. Current contents of the processor registers (including the program counter) saved to memory
4. Interrupt’s origin identified and appropriate ISR is called
5. All other lower-priority interrupts put on hold to allow the ISR to finish running
6. Program counter is updated with the address of the first instruction of the ISR
7. ISR executes
8. Processor registers are reloaded with the values that were saved to memory
9. Lower-priority interrupts that were put on hold are re-established
10. Program counter is set to point to the address of the next instruction that needs to be executed in the program that the processor was running when it received the interrupt

Question 26

How does a barcode reader work? (5 points)

Answer 26

1. A light from a laser illuminates the barcode
2. More light is reflected from the white areas than from the black bars
3. 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
4. These pulses are processed and converted to a binary number that represents the code
5. Most of them indicate that the barcode has been read successfully - beep or a light

Question 27

How does a digital camera work? (6 steps)

Answer 27

1. Shutter opens - light enters the camera through the lens
2. Image is projected onto the sensor at the back of the lens
3. Picture broken up into millions of pixels and tiny sensors measure the brightness of each pixel
4. A Bayer filter is used to determine the colour of each pixel - only intensity of red, blue or green light is measured at each one
5. The data is turned into electrical signals and stored as binary
6. Digital image recreated

Question 28

What are the 5 steps a printer takes to print a page?

Answer 28

1. Bitmap image of the page created
2. Negative charge is applied to the print drum and laser is used to change the polarity on the drum to be in line with the bitmap image.
3. Drum is exposed to positively charged toner that is attracted to the negatively charged areas on the drum
4. Sheet of paper is passed under the drum and the toner is transferred onto the paper
5. When the toner application process is complete, it passes to a fusing (heating) stage to ensure that the toner sticks to the paper

Question 29

How does a magnetic disk work? (3 points)

Answer 29

To read and write data, the device has two mechanical arms, one for each side of each platter

At the end of the arm is a read-write head with a tiny magnet - sweeps across the tracks while the sectors pass underneath the head as the disk rotates

Allows any block to be accessed directly, without having to read through the entire contents of the disk

Question 30

How does an SSD work? (4 points)

Answer 30

Made up of a controller and a bank of millions of NAND ‘flash’ memory cells

Each cell has a floating gate transistor that allows an electrical charge to be trapped

Charged cell represents a 0, cell with no trapped charge represents a 1

Cells are organised in terms ofpages- cell within a grid - andblocks - rows of cells

Question 31

What are the 2 advantages and 2 disadvantages of SSDs?

Answer 31

Advantages:
- Speed of access outperforms magnetic or optical devices
- Mechanical failure is far less likely

Disadvantages:
- Limited lifespan
- More expensive per GB than magnetic storage

Question 32

How is an optical disk read? (4 points)

Answer 32

The data is read serially, and the disc rotates at varying speeds, depending on the data being read — it rotates faster nearer the centre

Light from a laser beam is shone onto the surface of the disc

The light from the beam reflects off the aluminum layer and hits an optoelectronic device that detects any changes in light.

The pits and lands reflect different amounts of light, and the electronics in the drive interpret the changes as data bits.

Question 33

How is an optical disk written to? (3 points)

Answer 33

Laser in the optical disc drive is used at a higher intensity (than that used for reading)

The light will ‘burn’ tiny dots into the chemical layer to alter the amount of light reflected - creates a readable surface representing binary values

With rewritable (RW) discs, the chemical dyes that are used allow the burning process to be reversed - data can be erased and the disc can be written to again