DATA STORAGE UNITS

Bit: (BInary DigiT)

Is a single digit from a binary number, it is either a 0 or a 1. It is the smallest unit of storage since
all data is stored as binary codes.
Byte: is a group of 8 bits, representing a single character. This is normally the smallest grouping
used by a computer.
Kilobyte (KB)
(In computer storage terms symbols K, M, G, and T have particular values related to the powers of
2, not to the powers of 10)
A kilobyte is 1024 bytes which is 220 bytes.
Megabyte (MB)
One thousand kilobyte is megabyte.
Gigabyte (GB)
One thousand megabyte is a gigabyte. It is 230 = 1073741824 bytes.
Terabytes (TB)
One thousand gigabyte is a terabyte. It is equal to 240 which is 1099511627776 bytes.
Key storage terms:
1. Write: means to move or send data to a storage device.
2. Read: means to move/get/retrieve data from a storage device.
3. Read Only: means that data or programs in the storage device can only be copied
or moved but cannot be changed (written to).
Difference between primary storage and secondary storage

Primary memory Secondary memory

Data is read and written quickly Slow to read and write data
Located very close to the processor, so that signals They are located away from the processor.
take the shortest possible time in coming from and They are in separate units connected to the
going to the processor processor by cables
Primary storage only provides small amount of Secondary storage provides large amount of
storage space for data and instructions storage space
It is more expensive to buy It is less expensive, when compared to the
primary storage
Example: Random Access memory, Read Only Hard disk, flash disk, and Optical disks
memory

Why do we need a secondary storage?

Permanent storage: Primary storage loses data and programs when the power is interrupted.
Large amounts of data: Primary storage can hold limited amounts of data.
Inexpensive: It is comparatively inexpensive to buy compared to primary storage.

Disadvantages
The transfer of data from secondary storage to the CPU is much slower than from primary storage

ROM (Read Only Memory)

It is a type in which the contents may be read but cannot be written to by the computer system. It is
used to hold programs and associated data for building into computers. Software in ROM is fixed
during manufacturing.
Usually, the data held on ROM will be the software that tells the computer how to load the
operating system (called the boot program). Since data is permanently stored on ROM it will still
stay stored even if power is switched off. ROM is often called as non-volatile memory
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PROM (Programmable Read Only Memory)
It is a type of ROM which is manufactured as an empty storage array and in later permanently
programmed by the user. Data and instructions are programmed only once into the memory chip.

EPROM (Erasable PROM)

It is a type of PROM whose data can be erased by a special process and can be reprogrammed,
e.g. by the action of ultra-violet light, so new data can be written as if it were a new PROM.

EEPROM (Electronically Erasable Programmable Read Only Memory)

It is a type of ROM that allows the stored data or program to be erased electronically. The
advantage of EEPROM is that it does not have to be removed from the computer to be changed.

Temporary Storage
Buffer: A buffer is a temporary holding space for data moving between the CPU and input
devices, such as keyboards and printers. It is used to compensate the difference in speed between
CPU and peripherals. The input/output devices operate much more slowly than the CPU and
buffers are used to collect or send data at the speed of the input or output device.

Spooling: Spooling is the temporary storage of input or output data on magnetic disk or tape as a
means of compensating for the slow operating speeds of peripheral devices or queuing output from
different programs to one device, such as a printer.

Printer spooler: It is a temporary storage area which stores data ready to be printed. Each set of
data to be printed is known as a print job. When the print job has been received from a program
and is complete, the printer spooler can send this data to the printer. Several programs or terminals
can send data at the same time for printing, because the data is stored in files by the printer spooler
and sent to the printer when appropriate.
The print queue is a list of work waiting to be printed by a printer spooler.

Virtual memory: It is a method of extending or increasing Immediate Access Store by using

backing storage. Virtual memory is used when sufficient Immediate Access Store is not available.
Part of the hard disk is allowed to be used as if it were Immediate Access Store.

Cache memory: Some computers use an area of high-speed RAM memory called cache memory,
between the CPU and Immediate Access Store. If the required data or instructions is found in
cache, a program will run faster than if the information has to be retrieved from the Immediate
Access Store or secondary storage.

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STORAGE MEDIA AND STORAGE DEVICES
(SECONDARY STORAGE OR AUXILIARY STORAGE OR BACKING STORE)
Secondary Storage, also called auxiliary storage, is like a filing cabinet in which data are stored
until you need it.

The term storage media refers to the material (tapes or disks) on which data are stored.

A storage device is the equipment that writes the data to or reads the data from the storage
medium.

Main secondary storage devices are disk drives, and tape drives.
Disk drives include floppy disk drive, CD-ROM drive, DVD drive. A specific drive is needed to
read or write data into a specific medium.

MAGNETIC DISK STORAGE

Magnetic disk is the most widely used storage medium for all types of computers. Magnetic disk
offers high storage capacity, reliability, and capability to directly access stored data. There are
several types of magnetic disk including diskettes, hard disks, and removable disk cartridges.
Hard Disks
Hard disks provide larger and faster storage capabilities than diskettes. Hard disks consists of one
or more rigid metal platters coated with an oxide material that allows data to be magnetically
recorded on the surface of the platters. These diskettes are permanently mounted inside the
computer and are not removable like diskettes. However, you also get removable hard disks which
can be taken out of the computer and stored in a safer place.

Advantages of Using a Hard Disk on a Personal Computer

A hard disk drive on a personal computer provides many advantages for users. Because of its large
storage capacity, a hard disk can store many software application programs and data files. When a
user wants to run a particular application or access a particular data file on a hard disk, it is always
available. The user does not have to find the appropriate diskette and insert it into the drive. In
addition, the faster access time of a hard disk reduces the time needed to load programs and access
data. The typical access time of a hard disk for a personal computer is between 15 and 80
milliseconds.

Floppy disk drives:

These can be found on most microcomputers and accept the usual 3.5 inch floppy disks. High
density disks for a PC hold 1.44 MB of data (enough to store about 350 pages of A4 text).
Uses:
Floppy disks are useful for transferring data between computers and for keeping a back-up of small
files.
Advantages/disadvantages:
They are easily physically damaged if unprotected and magnetic fields can damage the data.
They are relatively slow to access because floppy disks rotate far more slowly than hard disks.
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Magnetic Tape
A type of digital storage media, magnetic tapes come in reel-to-reel as well as cartridge format
(encased in housing for ease of use). The two main advantages of magnetic tapes are their
relatively low cost and their large storage capacities (up to several gigabytes). Magnetic tapes
provide sequential access to stored information, which is slower than the random access of
magnetic disks. Magnetic tapes are a common choice for long-term storage or the transport of
large volumes of information.

Uses:
Magnetic tape can be used for permanent storage. Tapes are often used to make a copy of hard
disks for back-up (security) reasons.

Advantages/disadvantages:
Accessing data is very slow and you cannot go directly to an item of data on the tape as you can
with a disk. It is necessary to start at the beginning of the tape and search for the data as the tape
goes past the heads (serial access).
Magnetic tape is relatively cheap and tape cassettes can store very large quantities of data .

Compact Disc
CD's hold large quantities of data (700 MB) in the form of programs, text, sound, still pictures and
video clips. They are also known as optical disks because the data is read by a laser beam
reflecting or not reflecting from the disk surface.
CD's are available in two formats: CD-R and CD-RW.
CD-ROM's - ROM means Read Only Memory and this means you can only read from the disc, not
write or store data onto it. This is the most common sort of CD available and is the way most
software programs are now sold. The other one is CD-RW, where you can Read and Write. You
need to have a CD-Writer for writing on to a CD.
It is much faster to access data from a CD than a floppy but it is currently slower than a hard disk.

A CD recordable (CD-R) drive has a read/write capability. Using this device you can write around
700 MB of data in 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 disk called CD-RW. With suitable drives,
these disks can be written, erased and rewritten. They are more expensive than CD-R disks.
There are many useful features of CDs:
 they are easy to handle and more durable
 they can be used for multimedia presentations
 has a large storage capacity. A single CD can hold around 700 MB of data; hundreds of
floppy disks would be needed to store the same amount of data
 they can store text, sound, graphics and video.

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Digital versatile disks (DVDs)
1. DVD-ROM Drive: Similar to a CD-ROM drive, a DVD Drive reads CDs, CD-ROMs, and
the newer DVDs. The acronym DVD originally had no meaning, but has since been
referred to as Digital Video Disk and Digital Versatile Disk.
Size: 3.4 gigabytes

Advantages of DVD over CD

DVD CD
1) Holds more data, up to 3.4 gb 1) Holds less data up to 700 mb
2) Can hold full length movies and can be 2) More expensive to buy
double sided for additional storage 3) Access to data is faster
3) It is less expensive to buy
4) Access to data is slow

Memory sticks

Memory sticks are small, lightweight, removable and rewritable storage medium. They are
available in different sizes and are widely used this days.
Advantages
They are more compact and easily portable
Generally faster to read and write data when compare to other secondary media
Hold more data, and are more reliable (due to both their lack of moving parts, and their
more durable design) than other movable disks such as floppy disks and CDs.
Can easily connected to any of the PCs using a USB connection