Computer Organisation and Architecture

Question 1

The processor

Answer 1

The computer processor executes program instructions to run applications

Question 2

Main memory

Answer 2

The main memory is used to store program instructions and and frequently used data

It is much faster than secondary storage so storing frequently used data in RAM or ROM helps the
processor to execute instructions quickly

Question 3

RAM and ROM

Answer 3

RAM is Random Access Memory - it is volatile and can be accessed directly with the same access time for all read/write operations
- Its used as a store for currently running programs

ROM is Read Only Memory - it is non-volatile, meaning its contents aren’t lost when the power supply is turned off
- Its used to store programs and data that are needed as soon as the computer is turned on

Question 4

Buses

Answer 4

Buses are parallel wires that transfer signals between the internal components of the processor

Question 5

Address bus

Answer 5

The address bus is a one way bus used to transfer memory addresses and I/O locations

It goes out of the processor to other components

Wider address bus = more memory can be accessed as once
Each additional wire doubles the number of possible addresses that can be accessed

Question 6

Data bus

Answer 6

The data bus is a two way bus that transfers between the various components

It is one way out of the input controllers to other components and is one way into output controllers from other components

Wider data bus = more data can be transferred at once

Question 7

Control bus

Answer 7

The control bus is a two way bus that transports control signals such as memory read/write, interrupts and timing signals

Question 8

The stored program concept

Answer 8

The stored program concept states that:

• A computer program must be resident in the main memory to be executed
• Machine code instructions are fetched from the main memory, decoded and executed by the processor one by one in sequence
• The program in main memory may be replaced with another program at any time

The processor is instructed to perform arithmetic and logical operations - these are represented by machine code instruction

Question 9

Processor components

Answer 9

• Arithmetic logic unit = Performs arithmetic and logical operations on data
• Control unit=
  Controls the various components of the
  processor, as well as the fetch-decode-execute cycle
• The clock = Supplies a timing signal at a fixed rate. These signals are used to regulate the rate at which instructions are executed and to sync the operation of computer components

Question 10

Registers

Answer 10

Registers are very fast memory locations inside the processor or I/O controller

General purpose registers don’t have a specific role and can be used as storage for any data that is
required by instructions during execution

Dedicated registers do have a specific role

Question 11

Dedicated registers

Answer 11

• Program Counter = Holds the memory address of the next instruction to be fetched from main memory
• Memory Address Register = Holds the address of the memory location currently being accessed by the processor
• Memory Buffer Register = Holds the data item being transferred to and from the memory location being accessed by the processor
• Current Instruction Register = Stores a copy of the current instruction, both operand and op-code while it is being decoded and executed
• Status Register = Holds condition codes to indicate the outcome of operations

Question 12

Fetch-Decode-Execute cycle: Fetch

Answer 12

1) The content of the PC is copied to the MAR

2) The content of the MAR is transferred to main memory by the address bus

3) The instruction is sent from main memory to the MBR by the data bus

4) The PC is incremented by one

5) The content of the MBR is copied to the CIR

Question 13

Fetch-Decode-Execute cycle: Decode

Answer 13

1) The content of the CIR is decoded by the control unit

2) The decoded instruction is split into two parts: opcode
and operands

Question 14

Fetch-Decode-Execute cycle: Execute

Answer 14

1) Any data required by the instruction that isn’t present in registers is fetched

2) The instruction is carried out

3) Results of any calculations are stored in general purpose registers or main memory

Question 15

Interrupts

Answer 15

An interrupt is a signal from some device that is seeking the attention of the processor

They can be caused by program errors (arithmetic overflow) and I/O controllers (data transfer completion)

When an interrupt occurs, it is detected as a change in the content of the status register
between the execute and fetch stages of the fetch-execute cycle

Question 16

Dealing with interrupts

Answer 16

If an interrupt has been detected:

1) The contents of the program counter are saved in a stack

2) The contents of all the other registers used by the current program are also saved in a stack

3) The source of the interrupt is determined

4) Lower priority interrupts are disabled to stop them interrupting the interrupt

5) The program counter is loaded with the start address of the relevant interrupt service routine

6) The interrupt service routine is executed

7) Once the ISR is complete, the saved register values (other than the PC) belonging to the interrupted program are copied back into the relevant registers

8) Interrupts are re-enabled

9) The saved value of the PC is restored to point to the next instruction of the interrupted program to be fetched and executed

Question 17

Processor instruction set

Answer 17

The processor instruction set is the set of bit patterns / binary codes for the machine operations that a processor has been designed to perform

A machine code instruction is divided into the opcode and the operand - the opcode is the instruction to be executed while the operand respresents a data item or the address of a data item

Question 18

Addressing modes

Answer 18

The opcode can be subdivided into a part that represents how the operand should be interpreted called the addressing mode

• Immediate addressing = Where the operand is the actual value to use
  The # symbol is usually used to show that immediate addressing is being used
  (ex: MOV R2, #20)
• Direct addressing = The operand is the memory address of the location containing the data or where the data is stored

Question 19

Factors affecting processor performance

Answer 19

• Number of cores = The more cores a processor has, the more instructions can be executed at the same time, since each core can run independently. Tasks can be split up between cores
• Cache memory = The more cache a processor has, the more information it can store and the more time it can save in fetching information from main memory.
• Clock speed = A processor’s clock speed relates to the frequency of the pulses generated by the system clock. The higher the frequency, the more cycles of the fetch-execute cycle can be completed in the same period of time.
• Word length = A word is a group of bits that is treated as a single unit by a processor.
  Words can be used
  for representing both instructions and data. The length of a word is the number of bits that are assigned to it, with higher word lengths allowing for more bits to be transferred and manipulated as a single unit

Question 20

Factors affecting processor performance - Buses

Answer 20

• Address bus width = The wider the address bus of a processor, the more memory locations the processor can address simultaneously
• Data bus width = Increasing the width of the data bus increases the volume of data that can be transferred
  over the bus at any one time.
  A wider data bus allows the processor to fetch more data from main memory in one cycle of the fetch-execute cycle, reducing the number of cycles required to fetch large volumes of data

Question 21

Barcodes

Answer 21

A barcode is a diagram that consists of black and white lines that contain information, usually about a product

1D barcodes are straight black and white lines while 2D barcodes are called QR codes and store more information but needs more processing to extract the information

Question 22

Barcode readers

Answer 22

A barcode reader uses a laser and a light sensor to read barcodes

The barcode laser is directed to the barcode - the light reflected by the barcode reenters the barcode reader and is processed to create an electrical signal that represents the contents of the barcode

White bars reflect the most light while black light reflects the least light - this creates a pattern of reflection that is processed

Question 23

Digital camera

Answer 23

Digital cameras consist of a lens and light sensors

When a photo is taken, the shitter opens and lets light enter the camera.

The light hits the sensors at the back of the camera - each sensor converts the light into an electric charge that builds up on the cells of the sensor; each cell represents a pixel in the image.

Once the photo has been taken, the charge in each cell is measured and converted to a digital value which is processed by the camera and stored as a digital image

Two types of camera sensors, CMOS and CCD sensors

CMOS sensors take high quality images but use a lot of power

CCD sensors take lower quality images but use less power

Question 24

Laser printer

Answer 24

A laser printer uses powdered ink and a magnetically charged drum to produce high quality printing at high speeds

Printer software produces a bitmap image of what needs to be printed - the drum is then positively charged.
A laser is bounced off a mirror and shone onto the drum - this creates an impression of the image in electrical charge on the drum

This laser causes the areas it touched to lose their charge
The negatively charged toner particles are attracted to the positively charged parts of the drum - the toner is then applied to paper by the drum and fusers that use heat and pressure to fix the toner onto the paper

Question 25

RFID

Answer 25

RFID involves electronic tags and is a method of transferring information wirelessly

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

Passive RFID is where the tag has no power and relys on the power of a nearby RFID reader to work

Active RFID is where the tag has a battery - this tage actively transmits a signal that RFID readers can pick up

When an RFID tag is scanned, the reader emits radio waves which are picked up by the
tag’s antenna
The power induced in the tag’s antenna from these waves is enough to power the chip which then uses its antenna to emit its own radio wave, which contains the information held on the chip.
This wave is picked up by the reader which decodes the information and returns the information to a computer

Question 26

Secondary storage

Answer 26

Secondary storage is used to store files and applications - they are non-volatile and store data even when power is gone

Includes hard disk drives, solid state drives and optical disks

Secondary storage is needed since it retains data when the computer is switched off

Question 27

Hard disk drives

Answer 27

HDDs use rotating platters that are made with a magnetic material

An arm with a read/write head floats above each platter - this head polarises parts of the platters into north or south states that represent 0 and 1

Disks are divided into circular tracks and each track is divided into sectors

These disks spin rapidly thousands of times a minute, which allows for good read/write heads

Adding more platters and decreasing the width of tracks are two ways in which hard disk drives can be kept the same size while increasing their capacity

• High capacity, normally between 500GB to 5TB
• Good read/write speeds
• High latency
• Not portable - bulky due to multiple platters + arm with R/W head
• High power consumption - moving parts
• Suitable for desktops and servers

Question 28

Solid state drives

Answer 28

SSDs consist of NAND flash memory cells and a controller that manages the data on the drive - data is stored on SSDs in pages, which are combined to form blocks

The NAND memory cells are made of floating gate transistors which stores information by trapping electrons - the presence and absence of electrons indicates 0 or 1

SSDs aren’t capable of deleting data like HDDs - SSDs instead mark data as invalid.
The data stays on the drive until it is overwritten by new data

• Low capacity; usually 1TB or below
• Very high read/write speeds
• Very low latency - no moving parts
• Portable - lightweight
• Low power consumption - no moving parts
• Suitable for laptops and phones

Question 29

Optical drives

Answer 29

Optical disks include CDs, DVDs and Blu-Ray disks - they can either be read-only or rewritable

These disks use lasers on a reflective surface to read/write data

For read-only disks (CD-R, DVD-R, Blu-Ray), the laser burns pits and lands into the reflective surface to write data onto the disk - this data is permanent since the surface is permanently deformed

When reading the disk, a low power laser passes over the surface of the disk - this laser is reflected back differently depending on whether it passed over a pit or not. This pattern of reflections is read as 0s and 1s

For writable disks (CD-RW, DVD-RW), a photosensitive dye is used to rewrite data on the disk - the laser changes the dye it touches and changes how reflective it is, which writes information onto the disk

• Very low capacity
• Low read/write speeds
• High latency
• Very portable but prone to damage
• High power consumption
• Suitable for sharing small volumes of data