size of the main store or RAM is usually quoted in terms of the number of words which can be
accommodated simultaneously, as this is an indication of the power of the computer. A minimum of
about four thousand (4K) words is needed for even simple scientific applications, and microcomputers
can now be purchased with up to 2 gigabytes of RAM, whilst mainframe computers are very much
larger. Additional memory space (called backing store on mainframe computers) is available in the
form of magnetic tapes or disks and is used for long-term storage of programs and data. It is virtually
unlimited in size but it is relatively slow to access. Disk storage is much more widely used than tape,
especially for microcomputers. Rigid (hard) 15''–16" diameter disks with capacities of 1000 megabytes
and upwards are used in mainframe computers whilst mini- and microcomputers employ 8" hard or 8"
and 3 1/2" mini 'floppy' (flexible) disks with capacities ranging from one to several hundred megabytes
for programs and data depending on their size and the density of the stored information.
Two of the most recent developments in disk storage are the compact disk ROM (CD-ROM), which is
similar to an audio CD and read by a laser beam, and the magneto-optical disk, a hybrid
magnetic/optical storage medium which can be read or written to (R/W). The current storage capacity
of a CD-ROM is 750 megabytes and that of a DVD is 100–250 megabytes but will ultimately be
considerably more. Both can accommodate photographic quality images and/or complete libraries of
data (databases).
Input/output (I/O) devices enable the user to communicate with a computer, and instruments to be
interfaced with it. Visual display units (VDU), printers, plotters and magnetic tape and disk units are
the principal I/O devices. Although computers use machine code in all their operations, software for the
user is written in a more intelligible format known as a high-level language, e.g. BASIC, FORTRAN, ALGOL,
PASCAL, etc. These take the form of abbreviated or shorthand instructions in English which the computer
has to translate into machine code. The translation step involves the conversion of each alphanumeric or
other character in the user's program or data into a corresponding 8-bit number between 0 and 255 (the
ASCII code system). ASCII coded characters are then converted into machine code before execution.
High-level languages have been devised with particular types of application in mind, and BASIC in its
various forms has been the most widely used in the context of analytical applications.
Mini- and Microcomputers
The distinction between mini- and microcomputers is becoming essentially one of size and price.
Minicomputers, which use 16- or 32-bit words, had much larger memories than microcomputers and
could be used for the control of several laboratory instruments on a time-sharing basis. However,
microcomputers are becoming ever more powerful. Although some still use 8-bit words, 16-bit and 32-
and 64-bit word machines are becoming stan-