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BACKING STORES
We indicated earlier that a
computer's memory is limited in size, is needed as a working space for the
current program, and only retains information on a temporary basis. However
computers may often work on vast amounts of data and backing stores are used to
retain the data on a permanent basis. Information stored on these devices can be
retrieved and transferred speedily to the CPU when it is required.
Several different devices can
provide this additional storage space, but the one selected will depend mainly
on how the information needs to be accessed. There are two methods of access,
serial and direct. Information on a serial device can only be considered in the
same sequence in which it is stored. This would be suitable, for example, for
dealing with a mailing list where each address needs to be accessed in turn.
However, should an address be required out of order, it can only be retrieved by
searching through all those addresses which are stored before it. Frequently we
need to access information in a more direct manner than serial devices allow.
For example, at any given moment in a bank, some customer will be requesting
details about his account. Backing storage devices exist which permit access to
individual information in this more direct or immediate manner. These direct
devices are also called random access devices because the information is
literally available at random, i.e. it is available in any order.
Magnetic tape provides only serial
access. It can be referenced many times without the need for replacement. In
addition, information can be erased by recording new information in its place.
The tape has a ferromagnetic coating on a plastic base, is usually 1/2 in. wide
and comes in reels of 50 to 2400 feet. It is similar to the tape used on a tape
recorder except that it is of higher quality and more durable.
Information is retained on the
tape in the form of magnetized and non-magnetized spots (representing 1's) which
are arranged in tracks, typically seven or nine, running the length of the tape.
To represent a character in tracks, special codes are need, just as they are for
paper tape. Information recorded on magnetic tape is stored in varying
densities, typically 556 characters to the inch, or 800 or 1600 or even as dense
as 6250 characters to the inch, with the higher densities applicable to the more
modern systems using nine-track tapes.
Turning to the tape drive itself,
it can be seen that the tape runs from a supply reel to a pick-up reel via two
vacuum channels and between a set of read/write heads. The two vacuum channels
are designed to take up slack tape, acting as buffers to prevent the tapes from
snapping or stretching when starting from a stationery position or slowing down
from full speed. The read/write heads are present either to access information
on the tape. They are a single unit, made up of one read/write head per track.
Even though information can only
be accessed serially, magnetic tape is very widely used. Frequently it is
necessary to copy information and to retain it in the same order for use on
another occasion. Tape is ideal for this purpose as it is cheap and the transfer
rate to and from the computer's main memory are relatively fast. A reel of tape
is also a convenient way of carrying information from one place to another, i.e.
transferring information between computer systems which are not linked together.
In addition, tape is widely used to back-up information on magnetic disk and
there is increasing use of cartridge tape systems for this purpose.
Besides acting as an area of
secondary storage, magnetic tape is also an input/output medium in its own
right. Information is input to the computer from the tape for processing and
information is output to tape where it resides until it is needed again or until
it becomes redundant and is erased.
This device has direct access. In
shape, a disk resembles an LP record. A disk pack consists of a number of these
disks, six or more, monted about half-an-inch apart on a central hub which
rotates, spinning the disks at speeds of 50 or more revolutions a second.
Information is recorded on both sides of each disk as a series of magnetized or
non-magnetized spots, i.e. similar to magnetic tapes.
Information is stored on tracks
arranged in concentric circles, with each character represented by a pattern of
bits in sequence on one track. Although varying in length, each track contains
the same number of characters, which means that tracks on the outer reaches of
the disk are less densely packed with characters than those towards the centre.
Each track and sector address (76;5, that is 76th track and 5th
sector).
The diameter of a standard sized
disk is 14 in. and there may be several hundred tracks per surface, each with a
storage capacity of thousands of characters. Disk packs are potentially very
high capacity storage device typically in the range 20 t0 1000 megabytes (M
bytes).
The disk pack on some disk storage
devices is permanently fixed in position, whilst on other the pack can be
removed and replaced by another in a matter of seconds. Not all disks are
stacked in packs. A single removable disk is generally known as a cartridge
disk. The facility to change packs or cartridges means that storage space can be
increased without the heavy expense of buying another complete device.
The are two types of read/write
head units for magnetic disk devices, a moving-head unit and a fixed-head unit.
In the moving-head unit, the head moves horizontally across the surface of the
disk so that it is able to access each track individually. There is a head for
each surface and all the heads move in unison. Information stored on the tracks
which constitute a cylindrical shape through the disk pack are therefore
accessed simultaneously, a significant factor in storage arrangements.
Exchangeable disk packs are only associated with moving-head units. In the case
of the fixed-head unit, there is one read/write head for each track, as a result
of which not head movement is needed and information is therefore traced more
quickly. The heads do not have direct contact with the surface but 'rest on a
cushion of air. The movement caused by the revolving disk forces the head to
'fly' 1/400th of an inch from the surface.
The time taken to access
information on these direct, or random, devices varies considerably, but the
fixed-head device is quicker than the moving-head device. As with magnetic tape,
information on magnetic disk can be accessed again When fresh data is recorded
it simply replaces the existing information.
There may be as many as 256 tracks
and 32 or more sectors.
The Floppy disk is a comparatively
new storage device developed in the early 1970s as a cheap and faster
alternative to storage on magnetic tape. It is a small, random access disk
which, like all secondary storage devices, can be used both for input and output
operations. The disk is made of flexible plastic and coated in magnetic oxide.
For protection it is normally contained within a plastic cardboard sleeve, often
referred to as a cartridge. The cartridge is readily loaded into, and
unloaded from a drive unit. Unlike the moving-head read/write mechanism on
conventional disk drives, the heads on a floppy disk unit make contact with the
disk surface, when reading or writing, and disk therefore get worn with constant
use.
There are two standard sizes, 8
in, and 51/4 in., frequently referred to as diskette and mini-floppy
respectively. Storage capacity is small compared with other conventional disk
devices but quite impressive for size. The capacity of an 8 in. diskette is
typically between 250 K bytes and 1.5 M bytes and the capacity of a 51/4 in.
mini-floppy is between 125 K bytes and 500 K bytes, depending on density. The
floppy disk is a low-cost device particularly suited to supporting personal
computer systems and for use with small business systems and word processing
systems.
Winchester Disk
The most significant development
in disk technology in recent years is the introduction of hermetically sealed
units, generally knows as 'Winchester
disk drives, in which the read/write heads are designed to take off and land on
the disk surface. The disk is coated with a special lubricant which reduces the
friction when the heads and the sealed chamber prevent contamination from dust
and other airborne particles. The technology enables greater precision of
alignment, an increase in the number of tracks on the disk surface and a higher
storage density per track. Winchester
disks are fast and highly reliable, yet low priced compared with conventional
hard disk devices. Because the units are sealed preventive maintenance is not
required.
Standard sizes are 51/4 in., 8 in
and 14 in., with 8 in. the most common. Storage capacities of 10, 20 and 40
megabytes are typical, with double capacity on dual disk drives. A magnetic tape
cartridge transportable magnetic tape form. Winchester
disks are used to support minicomputers and are competing with 'floppies' for a
share of the expanding word processing and business systems market.
Disks are getting better all the
time and research aimed at increasing capacity and reliability continues. The
cost of disk storage is decreasing significantly and 8 in.
Winchester
devices of up to 180 megabytes capacity are eagerly awaited.
In early computer systems punched
cards were the most common form of input but today the emphasis has switched to
magnetic and optical media. In office and business organizations, where
traditionally data to magnetic type or disk is now standard practice,
superseding the use of the traditional punched card. Three methods are
distinguishable:
Key-to-tape
Key-to-cassette/cartridge
Key-to-disk/diskette
When the information is eventually
transferred from these backing stores, the backing stores themselves become
'input' devices. From keyboard to computer is yet another communication path.
Key-to tape
A key-to-tape device, sometimes
referred to as a magnetic tape encoder, permits the recording of information
directly on magnetic tape. An operator, copying from documents, keys the data
electronically using a typewriter-like keyboard. The data is stored temporarily
by the device and typically displayed on a CRT for visual checking before being
transferred to magnetic tape.
Key-to-cassette/cartridge
Information can also be keyed
direct to small magnetic cassette tapes or cartridges and later transferred to
standard magnetic tape for processing. A cassette, typically two and a had
inches by four inches in size is capable of storing around 200 000 characters of
information. A typical cartridge is smaller in capacity, averaging around 30,000
characters, These key input devices are easy to use and compact, and are
therefore most suitable for the collection of data at source, for example at
places remote from the computer installation.
Key-to-disk/diskette
As with key-to-tape systems,
information is entered via a keyboard and a copy displayed on a CRT to allow a
visual check. A key-to-disk/diskette system normally comprises a minicomputer,
number of key stations (one or more for a diskette system, typically 8 to 64 for
a disk system) and a disk drive. The minicomputer is required to control the
input from the various stations, enabling the data to be held temporarily for
verification and editing before allocation to the disk store. The process of
keying to disk or diskette is more efficient and cost effective than punching
data on cards for card input and is now the preferred method.
Backing stores serve two purposes.
They supplement the internal memory of the computer when linked to the CPU, and
they also store programs and data for future use. It is important to appreciate
that information from backing stores has to pass into the internal memory (under
the direction of control) before it can be used. This means that the CPU handles
information passing to and from conventional input/output devices.
