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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 bitsI byte = 8 bitsI kilobytes (kb) = 1024 bytes1 Megabytes (Mb) = 1024kilobytes1 gigabytes (Gb) = 1024Megabytes1 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 Its temporary memory Read and Write data Data can be changed
or deleted It is a Volatile memory The instructions
written in Ram at the time of execution
ROM Its Permanent memory It can only be read Data can not change or
deleted It is non Volatile The instructions are
written into Rom at manufacturing time. (BIOS)
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Memorytypes
RAM ROM
SRAM DRAM PROM EPROM
Volatile Non-volatile
Semiconductor
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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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