IGCSE Computer Science

binary numbers are used in

computer systems

conversion from denary to binary and from binary to den

use place values

memory size of a computer is measured in terms of the number of

bytes

1 kilobyte is

2 to the 10 bytes

1 megabyte is

2 to 20 bytes

Or 1 kilobyte according to manufactures

1000 bytes

Increase from

kilto to mega to giga to tera

unit symbols

kilobyte-kB

megabyte, gigabyte,tera byte is- CAPITAL thenB

Each increase by

1000 so mega byte has a 1000 kilobytes so thus a 1000000 bytes and a giga byte has a 1000 mByte

binary registers

group of bits in computer memory, each bit does something different so if 1 then x does y, small piece of memory built into cpu, where values and instructions are temporarily held. Not primary or secondary, very fast read and write rate, comp systems use registers to hold values and instructions for processing to increase the speed at which they are processed

Why choose Hex

Quicker to read, easier/quicker to write in hex, less likely to make errors copying so easier to debug

Letters in Hex

A=10

B=11

C=12

D=13

E=14

F=15

Register

Temporary piece of storage, status = stores what happening for reading, control gives instructions for writing

MAC addresses

media access control address 12-bit identification number unique for each device. Unique number that identifies a device, manufacturer id and serial number of device, address is allocated by manufacturer

Defining colours in HTML

Uses hexadecimal when representing colours, combinations of hex codes are used to represent different colours

#FF 00 00 = primary red

#00 FF 00 = primary green

#00 00 FF = primary blue

Memory Dumps

If there is an error in program it is useful to carry out memory dump, this is contents of a section of computer memory, Contents of a number of memory location are outputted in Hex format, Software engineer can then look at memory dump and find where error lies. Using Hex makes it easier and faster because ^

Assembly language and machine code

Memory location can be addressed directly using assembly language or machine code, this means its easier for programmers to write the code because not using binary

sound, music, pictures, video, text and number are

stored in different formats

Musical instrument digital interface

MIDI file comprises of data stored as a list of messages telling device how to generate a sound or music. Software saves the messages as .mid file, when played on an electronic keyboard the keyboard's software follows the .mid fil instruction and produces music,

-doesn't contain actual music so file size is much smaller than MP3 or MP4 files, 1Mbyte file size to store one minute under MP3 and 10kbyte Midi

MPEG-3 files

uses audio compression technology to convert audio to MP3 format, normal music on cd is compressed by a factor of 10. Uses perceptual music shaping, which removes sounds the human ear cannot hear properly, if 2 sounds are played MP3 discards softer sound that can't be heardeg

MPEG-4 files

format which allows multimedia rather than just audio files. Work in similar way to MP3 but they store audio , video, photo and animation files. Work b compressing files without affecting the quality by making certain aspects of the file redundant

Jpeg files

image encoding system that compresses photographs and other images. Lossy format and makes use of certain properties of human vision to eliminate certain unnecessary info

Lossless file compression

breaking down into files of similar size for transmission or storage and then put back together again at receiving end so it can be used again

Lossy file compression

Lossy compression algorithm eliminates unnecessary bits of information. It becomes impossible to get back to the original file once this lossy compression algorithm has been applied and the new file stored.

Algorithm looks for redundancies in file and eliminates them to reduce file size

Lossy Compression mark scheme points

images contain less detail without noticeable degradation in quality

streams faster than lossless compression

if 2 sounds played at the same time, softer sound is removed

when decompressed some detail is lost and file is not exactly like the original

Lossless compression mark scheme points

original and decompressed file will be the same

for code, decompressed file and original file must be exactly the same otherwise file wouldn't work

after decompression original file is restored with no loss of data

Data Transmission

Transferring binary data from sender to receiver

Serial

Data transmitted one bit at a time over a single wire

Parallel

Data transmitted several bits at a time simultaneously over several wires.

Advantages of Parallel

Faster, works well over short

Advantages of Serial

Cheaper cabling, works well over long

Disadvantages of Parallel

Conductive wires close together cause interference degrade signal quality

Disadvantages of Serial

Slower, synchronisation problems, receiver can't distinguish characters as one sent after another

Asynchronous

Signals sent in previously agreed pattern. Data includes control bits which informs receiver when transmitted data start and ends. Inexpensive serial.

Uses of serial transmission in USB

asynchronous serial, 4 wires, 2 for data other for power and earth.

USB advantages

Automatically detected and configured upon first attachment, not possible for incorrect instalment and industry support.

USB disadvantages

Cable limited to 5m and transmission speed to 480Mbps.

Use of parallel transmission in Integrated Circuits

Internal circuitry of computer is made up of IC's, method of transmission is parallel, transmission channels are usually 18bit to 64bit in width, the wider the channel the faster the transmission rate. An internal clock inside the devise makes sure all transmitted bits arrives in sequence.

Synchronous

Transmission accompanied by timing signals ensure sender and receiver are in step. Receiver counts bits and reassembles the data. Faster than asynchronous because less bits sent.

Simplex

Data can only transmitted one way from sender to receiver.

Half-duplex

Data transmitted between both sender and receiver but only one way at a time.

Full-Duplex

Data transmitted between both sender and receiver simultaneously.

Paritiy Check Verification Technique

Check byte of data, performed hen data received, parity bit added, number of 1's counted to see if even, can be even or odd, if parity incorrect error is then detected,

Problems with parity check

When even number of bits have been transposed

Parity byte

Parity byte is also sent along with other binary numbers which uses a parity check for each number. Parity byte is the additional parity bits found from a vertical parity check on the binary numbers, sent to indicated end of binary block

Finding errors in parity bytes

See which rows have an incorrect parity and columns, find intersection of row and column and change to make correct parity.

Automatic repeat request verification technique

Uses acknowledgements, requests and time outs, its an error control protocol. Check is performed on receiving data. If error is detected, then resend request is repeated until data is sent correctly. Sends acknowledgment that data is received. If acknowledgment is not received in set time data is resent.

Checksum verification technique

value that is calculated from data, added to data, then recalculated after data is entered then digits compared to check for error

ARQ timeout

Stop and wait until ACK arrives. If doesn't arrive use timeouts, so keep retrying for a x amount of times before u retry

Mark scheme points for serial transmission

more accurate over longer distances and more reliable over longer distance

less chance of data being skewed out of sync because less chance of interference

cheaper connection only single wire needed

easier to collate bits together again after transmission, synchronised after transmission

Check digit

digit calculated from data, added to data, recalculated when data is entered, digits are then compared

Role of a browser

display web page

translates HTML document

Functions such as bookmarks, stores history

Identifies protocols such as SSL

Role of Internet service provider

provide user with access to the internet

customers normally pay a monthly fee for this server

Hypertext transfer protocol

set of rules for transferring files on the web, http was later developed to allow secured transactions on the web to take place. Protocol works by transmitting info using encryption.

https

indicates secure webpage

HTML

Hyper Text Mark up Language

Language that provides structure and presentation. Kept separate, easier to update features like colour then.

Creates webpages

Translated by browser to display webpage

Uses tags to display content, used to define colour, graphics and layout

Web-authoring language

HTML structure

How layout of content is displayed. Create layout of doc. Markup tags used to define structure of document.

HTML presentation

How content will be formatted. Format colour. style of document. Often stored in a file called CSS, which is linked to HTML document.

IP address

Identify a device on a network, allocated by the ISP, can be used in place of a URL. Can be IPv4 or IPv6. Can be static means it doesn't change each time it is connected to internet. Can be dynamic means it changes each time a device is connected to internet. Give example. INTERNET PROTOCOL address

Different between IP address and Mac address

MAC address rarely changes so that a device can be identified no matter where it is

url

Uniform Resource Locator, a resource path is sent to ip address found from domain

cookies

oacked of info sent by a web server to a web browser and then sent back by the web browser to the web server every time it accesses it.

cookie function

used to authenticate user "Welcome x" if you are not x log out

User tracking and maintain user preferences

although info is used to form an anonymous profile of the user they have been subject to privacy concerns

Protocol

http how data is sent and received

Domain

Names of server, same as web server name www.cie.org.uk

Resource path

same as file name

How is request to access webpage sent to server

URL contains domain name. ISP looks up IP address of company. Domain name is use to look up IP address of company. DNS - domain name server stores an index of domain names and IP addresses. Web browser then sends a request to that IP address.

How is data received from web server

Browser connects with server containing files for website, data for website stored in server. Server then sends data for website back to browser, server uses customer IP address to do this. Transferred files translated to HTML. Interpreted by browser- display page.

NOT logic gate

Output is equal to opposite of input

AND logic gate

Output is always 0 unless both inputs are 1

OR logic gate

Output is always 1 unless both inputs are 0

NAND logic gate

Output is always 1 unless both inputs are 1

NOR logic gate

Output is always 0 unless both inputs are 0

XOR or EOR logic gate

If two inputs are same then output is 0, if two inputs are different then output is 1

Von Neumann Model

Processor uses address bus to find out which address you need to read from in memory

RAM then uses data bus to transfer data to cpu which is then sent to peripherals which run data

Control Bus from cpu to i/o tells what peripherals should do

Address Bus

Carries location of next item to be fetched. its unidirectional

Data Bus

Transports data that is currently being processed, bidirectional

Control Bus

Carries signals that controls the actions of the CPU, either Unidirectional or bidirectional

What is the execute function in the fetch-execute cycle

Control unit passes the decoded information as a set of control signals to the appropriate components within the computer. This allows all necessary actions to be carried out.

What is the fetch function in the fetch-execute cycle

The next instruction is fetched from the memory address currently being stored in the program counter and then it is stored in the current instruction register (CIR). The PC is then incremented so that the next instruction can be acted upon

Fetch-execute cycle 1

The PC contains the address of the next instruction to be fetched

Fetch-execute cycle 2

The address contained in the PC is copied to the Memory Address Register via the address bus

Fetch-execute cycle 3

The instruction is then copied from the memory location in the MAR and is placed in the Memory Data Register

Fetch-execute cycle 4

The entire instruction is then copied from the MDR to the CIR

Fetch-execute cycle 5

The value in the PC is then incremented so that it points to the next instruction to be fetched

Fetch-execute cycle 6 (execute)

The address part of the instruction s placed in the MAR

Fetch-execute cycle 7 (execute)

The instruction is decoded then executed

Control Unit

Controls operation of memory, processor and I/O. Instructions are then interpreted, sends signal to other components telling them "what to do"

Intermediate access store

memory found inside a CPU, used to hold not only data but also instructions before they are processed.

Arithmetic Logic Unit

used to carry out calculations on data, values need to be placed in accumulator in ALU for calculations to be carried out. Carries out logic gate operations.

MDR

contents of the address are in the MDR

MAR

the actual address

Control unit

Controls operation of memory, processor and I/O. Instructions are then intepreted, sends signal to other components telling them "what to do"

Step 1- fetching the instruction

CPU fetches the necessary data and instruction and stores them in its own internal memory locations (IAS). To fetch the instruction the CPU uses the address bus, CPU puts address of the next item that needs to be fetched on the address bus. Data from this address is then moved from main memory into IAS in to CPU travelling along data bus.

Step 2 - decoding the instruction

Needs to udnerstand the instruction that has just been fetched.

Step 3 - executing the instruction

processes data, calculations carried out in ALU

2D scanner how it works

The document is illuminated by a bright light

A scan head moves across the page until all of it has been scanned

The produced image is sent to a lens

Image is focused onto CCD(charge coupled device)

CCD is made up of thousands of light sensitive elements which create an electric charge when light falls onto it thus converting it into an electric form

A digital image is created from this electric form

2D scanner how it works simple

illuminated, scan head scanning doc, scanned image sent to lens, focused on CCD, converted into digital image Reflected light is captured, through use of mirrors and lenses captured image is converted into a digital file. Producing a 2D digital image

2D scanner markscheme

uses light to illuminate document (shines light onto surface of document), scan head then scans document. Scanned image is then sent to lens, where it is focused on CCD (Charge coupled device) to be then converted into a digital image. Reflected light is captured, through use of mirrors and lenses captured image is converted into a digital file. Producing a 2D digital image.

2D scanner application

Used at airports to read passports

Scanning documents and digitizing them