Bits and Bytes in a computers memory

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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 0s, and
1s is called BINARY. Each 0 or 1 is a BInary
digiT or BIT for short.
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Basic Concepts A computers most basic element
is a switch, either off or on.
RAM or ROM switches
Written in a book machine code
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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!
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Storage Capacity
This is how much data that RAM, ROM, disks etc.
can hold
8 bits 1 byte 1024 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
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A disk is FORMATTED into tracks and sectors
This allows the FILE MANAGEMENT system to find
files on disk
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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
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How does information get stored on these lines of
"magnetism"? Look at the diagram
Disk READ HEAD
Surface of the disk
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.
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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
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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.
The number of characters for each record is
8201852252525 128 Hence the total number
of bytes required 128x20000 bytes 2560000
bytes 2560000/1024 Kb 2500 k
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The PC computer (and MacIntosh) computers use
directories to organise their filing system.
The situation can be represented in this diagram
This is called a Hierarchical Filing system.
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Interpreter Takes ONE line at a time Translates
it Executes it Gets next line
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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).

• ASSEMBLY
• Used by experts to get memory efficient code
• Translates directly
• 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