# Computer fundamentals units 3 and 4

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3. Data: representaion and structure

a. express numebrs in binary, BCD, octal and hex

Binary

128 64 32 16 8 4 2 1

BCD

Octal

split the bianary number into three groups of 3

2 1 4 2 1 4 2 1

Hex

split the binary into two groups

8 4 2 1 8 4 2 1

b. describe and use two's complement and sign and magnitude to represent negative integers

two's complement

example: write the number -90 as a 2's complement binary number in a single byte

-90 = -128 + 32 + 4 + 2 = 10100110

another example:  write the number -58 as 2's complement binary number in a sinlge byte

-58 = -128 + 64 + 4 + 2 = 11000110

sign and magnitude

if the first number is a 1 then the number is negative, if it is a 0 then the number is positive

and then the rest are the same

c. perform integer binary arithmetic, that is addition and subtraction

1 0 1 1 0 0 1 0

0 0 1 0 0 1 1 0

1 1 0 1 1 0 0 0

1 1 1

subtraction

d. explain the use of the code to represent a character set (ASCII, EBCDIC and UNICODE)

ASCII: is a 7-bit code that uses a single byte to represent a character ( the 8th but used to be used as error check but now it is used to provide a further 128 characters). It can only encode 256 different characters

UNICODE: uses 4 bytes, gives room for up to 4,294,967,296 characters

EBCDIC: uses one byte, but not the same encoding as ASCII.

e. describe manual and automatic methods of gathering and inputting data into a system

• voice recognition
• bar code:
• OMR
• OCR
• MICR
• touch screen
• chip and pin system: use a microchip embedded into the card to authenticate a user's digit PIN.
• sensors: devices that measure some physical quantity, converting the information into  human useable form.
• data logging: the collection of data from one or more sensors at a regular time interval.

f. explain the techniques of validation and verification, and describe validation test which can be carried out on data

validation: checking that data is reasonable and complete

• presence check
• length check
• format check
• range check
• existance check

verification: checking that data is consistent

• visually checking
• entering the data twice

g. describe possible forms of output

h. explain the procedures involving in backing up data and archiving

backing up: the regular, frequent (daily) copying of important data to an external device which is then often removed for safe keeping. this means that the data can be restored if the original data is corrupted and damaged in any way. normally copied to a DVDRW.

archiving: the regular but usually infrequent (anually) transfer