ComSci - P2

why is the hexadecimal number system used?

• used as a shorthand for binary since it is simple to represent a byte in two digit.

• fewer mistakes are likely to be made writing a hex number than a string of binary digits.

• easier for technicians to write and remember a hex number.

• colour codes in images often use hexadecimal to represent the RPG values

bit

a bit is a fundamental unit of information in the form of either a 1 or 0, used to present two electronic states: on or off

byte and nibble

a byte is 8 bits, a nibble is 4 bits

how many values can n number of bits represent

can represent 2ⁿ number of values

Ascii code

American Standard Code for Information Interchange

uses 7 bits, enough to cover the standard English language keyboard

extended to 8 bits to include symbols such as é 

Unicode

• several coding systems created, all incompatible with one another, multilingual data was increasingly used

• 16-bit code created, the first 128 bits were the same as ASCII

• uses more storage than ASCII, significantly increasing file sizes and transmission times

parity bits

additional bit used to check that the other bits transmitted are likely to be correct

using 7-bit ASCII, with an 8-bit system there was an extra bit available, this was used as a parity bit

computers use either an odd or even parity bit, used to ensure that the total number of 1s in each byte(parity included) equals an odd or even number

majority voting

system that requires each bit to be sent three times

if bit value is flipped erroneously during transmission over a noisy line, recipient would use the majority rule and assume that the two bits that have not changed are correct.

checksums

a mathematical algorithm that is applied to a ‘unit’ or packer of data e.g. a block of 256 bytes

data in the block is used to create a checksum value which is transmitted with the block. 

the same algorithm is applied to the block after transmission and if the two checksums match, transmission is deemed to be successful

check digits

similar to checksum, additional digit at the end of a string of other numbers designed to check for mistakes in input or transmission

printed books and other products have unique ISBN(international standard book number) or EAN(European article number), a 13-digit number which includes the calculated check digit and is printed with the barcode. First twelve digits are the unique item number and the 13th is the check digit