3.7 Fundamentals of Computer Organisation and Architecture

What does the processor do?

Executes program instructions in order to run applications.

What is main memory comprised of?

* RAM
* ROM

What is the role of main memory?

To store program instructuions and frequently used data

How does storing data and instructions in main memory over secondary storage help the running of the computer?

Main memory is usually much faster than secondary storage so storing in main memory helps the processor to execute instructions quickly

what is a bus?

series of parallel wires that connects internal components of a computer system, allowing signals to be passed between them.

what are the three examples of buses?

* address bus
* control bus
* data bus

what is the number of wires a bus have called?

its ‘width’

how do buses transmit data?

parallel data transmission

what does the width of a bus have a direct relationship to?

the number of bits that can be transferred simultaneously by the bus

what is the role of the address bus?

to transport memory addresses, specifying where data is to be sent to or retrieved from

what does increasing the width of the address bus do?

increases the range of addresses that it can specify

what does increasing the range of specifiable addresses do?

increases the computers amount of addressable memory

what does adding a single wire to the address bus do?

doubles the number of addressable memory locations

what is an addressable memory location?

a portion of memory that can be accessed by its address

what happens if there are not enough adressess

some portions of memory go unused

what does the data bus do?

sends data and instructions to and from difference components of the computer system.

what does increasing the width of the data bus do?

increases the volume of data that can be transferred over the bus at any one time

what does the control bus do?

* used to carry control signals that regulate the operation of the computer system
* also carries the computer’s clock signal

what are I/O controllers?

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

what are some examples of I/O devices?

* keyboards
* mice
* monitors

what is the key fact about Harvard architecture?

processor will use two separate main memory locations: one for instructions and another for data

What can Harvard architecture take advantage of that Von Neumann cannot?

Its memory can be read only memory if they do not want instructions to be altered

what is harvard architecture used in?

embedded systems like digital signal processing (DSP)

what is von ==neumann== architecture?

both instructions and data are stored together in the same memory

how does von neumann architecture perform in comparison to harvard architecture? why?

* often perform worse than
* same buses have to be shared for fetching both instructions and data

what is von neumann architecture used in everyday?

general purpose systems like laptops and smartphones

what is a computer that uses the stored program concept?

“serially fetching and executing machine code instructions stored in main memory by a processor that performs arithmetic and logical operations”

what does the s**tored program concept an**d the way that it stores program instructions in main memory allow?

one set of instructions to be switched out for another.

what does ALU stand for?

Arithemetic Logic Unit

rwhat is the tole of the ALU?

to perform arithmetic and logic operation

what are the operations the ALU performs?

mathematical operations like addition, logic operations include AND, OR and XOR.

what is the role of the control unit?

controlling the various components of the processor and the fetch-execute-cycle

what is the role of a register?

small storage locations used to hold temporarily with high read and write speeds

what are the two types of registers?

* general purpose
* special purpose

what are the five types of special register?

* program counter
* current instruction register'
* memory address register
* memory buffer register
* status register

what is the role of the program counter?

to hold the memory address of the next instruction to be executed in the fetch-execute cycle

what is the role of the current instruction register?

to hold the instruction that is currently being executed by the processor

what are general purpose registers role?

storage for any data that is required by instructions during execution

what are special purpose registers used for?

storage of specific information

what are the types of special purpose registers?

* program counter
* current instruction register
* memory address register
* memory buffer register
* status register

what is the role of a program counter?

to hold the memory address of the next instruction to be executed in the fetch-execute cycle

what is the role of the current instruction register?

to hold instruction that is currently being executed by the processor

what is the role of a memory address register?

to store the memory address of memory location that is to be read from or written to

what is the role of the memory buffer register?

to hold the memory location that has been read from or data that is to be stored

what is the status registers?

to contain a number of bits, the values of which can change to indicate the occurrence of an interrupt

what is the role of a clock?

generates a timing signal which changes a regular frequency

what is the signal of a clock used to do?

synchronise communication between the components of the processor and the rest of the computer system

what is the fetch-decode-execute cycle?

a continuous cycle performed by the processor

what happens in the fetch stage of the fetch-decode-execute cycle?

* next instruction to execute is retrieved from main memory
* the content of the PC is copied to the MAR
* content of MAR transferred to main memory by the address bus
* instruction is sent from main memory to the MBR by the data bus
* PC is incremented by one
* content of the MBR is copied to the CIR

what happens in the decode stage of the fetch-decode-execute cycle?

* content of the CIR is decoded by the CU
* decoded instruction is split into two parts: opcode and operands

what happens in the execute stage of the fetch-decode-execute cycle?

* any data required by the instruction that isn’t present in registers is fetched
* instruction is carried out
* results of any calculations are stored in general purpose registers or main memory

what happens between each execute stage and fetch stage of the cycle?

contents of the status register is checked for changes that could signify occurrence of an interrupt

what is a processors instruction set?

group of instructions that it can carry out

why may the instructions for one processor not be compatible with other processors?

each type of processor has its own instruction set

what are instructions usually stored in?

machine code

what are the two parts of machine code instructions?

* opcode
* one or more operands

what does opcode specify?

the type of operation that is to be carried out

what are operands?

the pieces of data on which the operation is performed

what is one bit in a machine code instruction usually assigned to?

the addressing mode in use

what are the two types of addressing mode?

* immediate
* direct

what happens in immediate addressing?

value specified in the operand is to be treated as the actual value

what is an example of immediate addressing?

if the operand were 18, the value used by the operation would be 18

what happens in direct adressing?

the value specified in an operand signifies a memory address

what is an example of direct addressing?

if the operand were 18, the value to be used by the operation would be whatever the content of the memory location 18 is

what is a logical shift?

operation performed on binary numbers which involves shifting all the bits in a number a specified number of positions to the left or right

what does performing a logical left shift do (once)?

double it

what does performing a logical right shift do (once)?

half it

what is an interupt?

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

what is an example of a hardware interupt?

the computer’s I/O controller informing the processor that the mouse has been moved or that a keyboard key has been pressed

what are some examples of interrupts that software can send?

unexpected errors like:

* division by 0
* stack overflow

how is an interrupt detected?

a change in the content of the status register between the execute and fetch stages of the fetch-execute cycle

how can interrupts be handled?

using the vectored interrupt method

what happens during the vectored interrupt method? what is this called?

* the processor stops executing the current program and places the content of its registers onto the system stack


* aka saving the “volatile environment”

what happens, when dealing with an interrupt, once the volatile environment is saved?

loads the appropriate interrupt service routine

what is an appropriate interrupt service routine?

a series of instructions for handling the interrupt that is specific to the type of interrupt

what happens, when dealing with an interrupt, once the processor finishes executing the interrupt service routine?

restores the volatile environment from the system stack and resumes execution of any programs that were running before the interrupt

what are the six factors that can effect the processor’s performance?

* multiple cores
* cache memory
* clock speed
* word length
* address bus width
* data bus width

how does the number of cores a processor has affect the performance of the processor?

* each core performs its own fetch-execute cycle independently of others
* different applications can be allocates different cores

what is a processor’s cache?

* small portion of incredibly fast memory
* read and write speeds far higher than HDD and SSDs

what is cache used for?

storing frequently used information and reduces time wasted in fetching the same information from main memory again and again

how does the amount of cache affect the processor’s performance?

the more information it can store and the more time it can save fetching information from main memory

what does a processor’s clock speed relate to?

the frequency of pulses generated by the system clock

how does a higher clock speed affect the processor?

the higher the frequency, the more cycles of the fetch-execute cycle cycle can be completed in the same period of time

what happens if the computer’s clock speed is increased too far?

errors can occur

what is a word?

group of bits that is treated as a single unit by a processor

what can words be used to represent?

* instructions
* data

what is the length of a word?

the number of bits that are assigned to it

how does a higher word length effect the performance of the processor?

higher word lengths allow for more bits to be transferred and manipulated as a single unit

what is the width of a bus?

the number of parallel wires that make up the bus

how does increasing the width of the address bus help the computer perform?

* increases the range of addresses it can specify
* increasing the computer’s amount of addressable memory

how does increasing the width of the data bus help the computer perform?

* increases the volume of data that can be transferred over the bus at any one time
* processor can fetch more data in one cycle
* reduces the number of cycles required to fetch large volumes of data

what is a barcode?

printed diagrams that consist of light and dark portions

what do barcodes contain? what is used to read this?

* information
* computer

what are the two main types of barcode?

* 1D
* 2D

how do 2D and 1D barcodes differ in terms of amount they contain?

2D barcodes contain more information in the same amount of space

how do 2D and 1D barcodes differ in terms of processing?

2D barcodes require more processing in order for the information to be extracted

what do barcode readers consist of?

* laser light source
* lens
* photodiodes
* mirror

what does a photodiode do?

turns light into electrical charge

how does a barcode reader work?

1. mirror directs light onto printed barcode'
2. light reflected by barcode passes through lens
3. light is incident on the photodiode
4. electrical charge measured and processed to form digital signal
5. represents content of a barcode

what portions of a barcode reflect the most light?

light portions