Transcript Chapter No 4 - Computing A level Paper 1

Storage of Data
 Instructions and data are held in main memory
which is divided into millions of addressable
storage units called bytes.
 One byte can hold one character
 One byte can hold 8 bits
 Computers work by using pulses of voltage
which represent either 1 or 0
 A low voltage mean 0 and high voltage mean 1
 Total bytes in main memory referred to as the
computer’s memory size.
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Bits
Computers use electronic circuits to
perform the tasks they are
programmed to complete.
These circuits can be in one of two
states - ‘On’ or ‘Off’.
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Memory Sizes measurement
I nibble
= 4 bits
I byte
= 8 bits
I kilobytes (kb)
= 1024 bytes
1 Megabytes (Mb) = 1024kilobytes
1 gigabytes (Gb) = 1024Megabytes
1 terabytes (TB) = 1024Gigabytes
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Memory ( Main store)
 Memory is the name given to the group of chips inside
the processing unit
 The data held in the memory is available to the computer
 Memory is used to hold following information
 Programs
 Input data
 Working Area
 Output data
 There are two types of main memory
 Ram (Random access memory )
 ROM (Read only memory )
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RAM & ROM
Ram
ROM
 Its temporary memory  Its Permanent memory
 Read and Write data  It can only be read
 Data can be changed  Data can not change or
deleted
or deleted
 It is a Volatile memory  It is non Volatile
 The instructions are
 The instructions
written into Rom at
written in Ram at the
manufacturing time.
time of execution
(BIOS)
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Semiconductor
Memory
types
Volatile
Non-volatile
RAM
SRAM
ROM
DRAM
PROM
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EPROM
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PROM & EPROM
 PROM
 Programmable read only
memory
 User can write instruction
only one time
 Instructions can not be
erased
 If there is error while
writing on PROM ,it
becomes unusable
 EPROM
 Erasable programmable
Rom
 User can write instruction
many times
 Instructions can be
erased
 If there is error while
writing on EPROM, it can
still be used again
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SRAM & DRAM
SRAM
 Static Random Access
Memory
 It can store its value
without any need to
refresh the data as
long as the power is
available
 It utilizes less power
 It is more expansive
DRAM
 Dynamic Random
Access Memory
 It has to be refreshed
after each read
operation.
 It utilizes more power
 It is less expansive
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Floppy Disk
 Used to transfer small amount of data between
computers
 Come with 3.5 inch disk drive
 Floppy disk is the hard case the disk inside in
floppy
 Magnetic disk store binary data as a magnetic
pattern on the disk surface
 It hold 1.44mb of data
 Before the data is stored on a disk the disk
needs to be formatted
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Hard drives
 Consist a several disks on a single
spindle
 Each disk surface able to store data
 Each surface can have its own
read/write head
 This enables the read/write heads to
operate simultaneously which means
that the data can be transferred more
quickly than by using the large disk
 Hard disk is available in all sorts of
sizes
 A typical computer will have a hard disk
capacity of around 20 GB
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Hard Disk Mechanism
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Magnetic Tape
Large spools of tape are used very large
computer systems that need to hold huge
amount of data
They are not as common as they used to
be
They look similar to audio cassettes
except they are larger
Their main use is for making backup
copies of hard disk
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CD-ROM Drives
 Use the same technology as music CD and CD
players
 The data is stored on the disk digitally and a
laser beam is used to read the disk
 Light is used to read the disk the data may be
packed closely and disk therefore has a huge
capacity
 Are included in multimedia system
 CD-ROMs can be filled with clip art
,encyclopedia ,photograph and all sort of data
 Are read only disk
 A CD holds around 600MB data
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CD-R
A CD recordable (CD-R) drive has a
read/write capability.
Using this device you can write around 15
minutes
CD-R can be used to back up hard drives.
Standard CD-R disks can be written to
only once but there is a type of disks can
be written, erased and rewritten
They are more expensive than CD-R disks
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Digital Versatile Disks (DVD)
 Are similar to CD-ROMs in that they look identical and
are used to store lots of date.
 Like CD-ROM they can be used to store computer
application software, multimedia programs and full length
movies
 This is because CD-Rom typically store 650 MB of data
whereas to a DVD stores between 4.7 GB
 DVDs also are used to store movies where they produce
much better quality pictures and sound compared to a
normal video tape.
 A film stored on DVD can even be made to offer the
viewer a choice of several languages in which it can be
played back
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Backup devices
 It is important to take regular backups of both
data and programs for security purposes.
 Floppy disks do not have a very high storage
capacity and so are seldom used for backup.
 Instead higher capacity and faster storage
devices are used and main ones are as follows
 Tape streamers
 Zip drives
 Jaz drives
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Tape streamers
 Are devices used to backup the data contained
on hard drives
Zip Drives
 Are useful for backing up hard drives
 They are high storage capacity floppy disks
which are slightly larger and twice as thick as
normal floppies
 They can typical store data 100 MB or 250 MB

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Jaz Drives
Is a removable disk drive which is very fast
at transferring data.
Each disk hold up to 2 GB of data
The higher data transfer rate means that
they can be used to backup data in a very
short time
They can be used for everyday storage
just like an ordinary disk drive
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