off = 0

on = 1

O1 1 1 1O O O

O 11 1 OO 1 O

Bits and Bytes in a computers memory

Inside the computer are millions of electronic switches. These are grouped together in bundles of 8. A switch can be either on or off. It can be seen then that a switch can stand for either a 1 when it is switched on or a 0 when it is switched off.The switches are gathered into bundles of eight, and by having them on and off different patterns can be made, e.g.:

The system of numbers, which has only 0’s, and 1’s is called BINARY. Each 0 or 1 is a BInary digiT or BIT for short.

Basic Concepts

A computer’s most basic element is a switch, either off or on.

1 0 0 1 0 0 0 0

0 0 1 0 1 1 1 0

0 1 0 0 1 1 0 1

1 0 1 1 0 1 0 0

1 1 0 1 1 0 1 0

0 0 0 1 1 1 0 1

1 1 1 0 1 0 0 0

1 0 0 0 0 1 0 1

1 0 0 1 1 0 1 0

0 1 1 1 1 0 1 1

0 0 1 1 0 1 1 0

1 1 0 0 0 0 1 0

RAM or ROM: switches Written in a book – machine code

Why 8 bits = 1 Byte?

8 different switches can have 256 different combinations.

For example A given ASCII code 65 = 01000001

This is enough to cover: 26 Upper case letters

26 lower case letters

10 different digits 0,1,2,…,9

All the punctuation marks on the keyboard e.g. !,”,£, |, * , #, etc.

4 or 6 bits would not be enough to cover all possibilities.

There are still some code numbers left over using 8 bits; these are used in programming for special effects. They are called CONTROL CODES.

We are all familiar with our characters in British English, but different countries use different character sets. Even America has slightly different characters. Think about Russian or Turkish!

8 bits =1 byte1024 bytes =1 kilobyte ( 1 Kb)1024Kb =1 megabytes ( 1 Mb)1024 Mb =1 gigabyte( 1 Gb)1024 Gb =1 terrabyte

Someone has said that the storage of the human brain is just over 4 Terrabytes

Storage Capacity

This is how much data that RAM, ROM, disks etc. can hold

file saved at different parts ofthe disk

file saved at track 2sector 0

1

2

0

A disk is FORMATTED into tracks and sectors

This allows the FILE MANAGEMENT system to find files on disk

1

2

0

Tape storage

This uses SEQUENTIAL ACCESS i.e. the system has to read from beginning to end of the tape to find records or data.

It is therefore slow. You would not want to use it at your home computer! Why use it?

Robust

High Capacity

Cheap

Easily stored

Disk storage uses DIRECT ACCESS, because the disk read heads go directly to the start of the file to read the file.

It is sometimes also called RANDOM ACCESS

Record Record Record Record Record Record 1 2 3 4 5 6

MAGNETIC TAPE

Microscopic 'magnets' withdifferent polarity.The drive head can detect these - and eg no change = 0, change = 10 1 0

How does information get stored on these lines of "magnetism"? Look at the diagram:

All data is stored as binary numbers and the different polarities (i.e. north and south pole) lead to the binary digits being laid down on disk. As the read head passes over the microscopic magnets, a change in polarity is a ‘1’, and no change in polarity is a ‘0’.

Disk READ HEAD

Surface of the disk

Example. A book has 345 pages. Each page has an average of 82 lines, 9 words per line and 7 letters per word.

How many bytes are required to store the book ? How many kilobytes is this?

Solution: 1 letters takes 1 byte;

no. of letters = 345 X 82 X 9 X 7=1782270 letters=1782270 bytes to store the book

to change into kilobytes divide by 1024

= 1782270/1024 Kb = 1740.5 Kb

Membership number <---8---->

Surname <--------20 letters---> Middle Initials <--5-->

First Name <------18---------->Age <-3->

Address 1 <-----------25------------>

Address 2 <-----------25------------>

Address 3 <-----------25------------>

Data stored in a database

Look at this record:

Example. How much storage space would be required for 20,000 records like the one above?

Solution.There are eight fields in each of the records.

Field Name Data Type Field Size

Membership Number

Alphanumeric

8

Surname Text 20

Middle Initials

Text 5

First Name Text 18

Age Number 2 (numbers are always 2 in Access)

Address 1 Text 25

Address 2 Text 25

Address 3 Text 25

The number of characters for each record is 8+20+18+5+2+25+25+25 = 128Hence the total number of bytes required

= 128x20000 bytes= 2560000 bytes= 2560000/1024 Kb= 2500 k

The PC computer (and MacIntosh) computers use directories to organise their filing system.

File 1

Hard Disk

File 2

File 1 File 2

File 1 File 2 File 1

File 1

Folder 1 Folder 2 Folder 3

Folder 4 Folder 5

The situation can be represented in this diagram:

This is called a Hierarchical Filing system.

100 perform_preparations200 while item NOT found do300 begin400 if value <> search 500 then600 process700 else800 increment900 end if1000 end1100 end while1200 process_results1300 print_table1400 end

100101101010100001100001010010111110001100110010110100101001011110010010101000001010010000111110

╬nterpreting:1 one line at a time2 translates AND executes3 NO machine code version4 errors detected early

T ra

n

s lat i o

n

execute

get next line

Interpreter

Takes ONE line at a time

Translates it

Executes it

Gets next line

Pro Con

Interpreter As it executes lines of code after it translates, it finds errors quickly

It does not produce object codeAs it re-translates each line of code inside a loop for example, it can be slow

Compiler Produces fast object code

Difficult to find errors

Assemblers. Assembly language is a LLL that can be used reasonably easily by scientists but is near to machine code (the actual language of the processor).

10010110101010000110000101001011111000110011001011010010100101111001001010100000101001000011001110

T ra

n

s l a t i o

n

LDA &C3AAADC &FF00STA &C3ABTAYBNE &FF0AADC #&222

Assembly program

ASSEMBLY:

1 Used by experts to get memory efficient code

2 Translates directly

3 Gives fast machine code version

4 Similar to compiler

These are location ADDRESSES in a number system called HEX

These are codes for binary commands