Computer Data Storage

موضوع: Computer Data Storage الثلاثاء يوليو 19, 2011 10:14 pm

The earliest form of magnetic mass-data storage was magnetic tape, which had
been originally developed for the analogue recording of sound waves (see p.
724). However, even with multi-track digital recording, which allowed the bits
encoding each character to be stored in parallel, tape had to be scanned
sequentially. Computers also needed rapid random access to small amounts of
data. Magnetic cores strung on a grid of wires gave the highest speeds, but were
very expensive on a per-bit basis (one bit per core); therefore, magnetic drums
were introduced for back-up mass memory. In 1956, IBM introduced RAMAC (Random
Access Method of Accounting and Control for Automatic Computers), the first
magnetic disk system. Up to 100 rigid disks were mounted on a common spindle
which spun at high speed; data were written and read (computerese for recorded
and reproduced) by magnetic-sensing heads flying in and out only microinches
above the disk surfaces. In 1962, IBM marketed the first cartridge magnetic
disks, which could be mounted on a disk drive for access, but removed for
storage—a similar system to that employed for magnetic tapes. About 1970, IBM
introduced a non-rigid magnetic disk which has come to be called the floppy
disk, or simply floppy. This has become the preferred medium for off line
storage of data and programs for small computers. The original floppies were 8in
(20.3cm) in diameter and could store 100,000 bytes (100 kbytes) on a side; the
encoding system, data density and file format were set by IBM, becoming a de
facto standard. However, when microcomputers became popular in the late 1970s, a
5 1/4in (13cm) version was introduced, soon dubbed the minifloppy.
Unfortunately, formats were never standardized, so that by the early 1980s there
were a hundred different ones: single-density, double-density, even
quad-density; and both single-sided and double-sided. Despite this diversity,
the 5 1/4in floppy has become the universal external storage medium for
microcomputers, providing from 200 kbytes to more than 1 megabyte (1 Mb) per
disk. In the early 1980s, further miniaturization took place: a 3 1/2 in (9cm)
diameter disk, sometimes called a microfloppy. These disks can store 1.5 Mb and
are more reliable because they are enclosed in a rigid case.

Also, in the
late 1970s, rigid disks of 8in (20cm), and then 5 1/4 in (13cm) diameter were
introduced to provide much greater mass-storage capacity for microcomputers.
They are called hard disks to distinguish them from floppies, and are usually
not removable, being housed in a dust-free casing. They spin at much higher
speeds than floppies, and their plated-metal surfaces are capable of much higher
recording densities, providing users with from 20 to more than 100 Mbytes of
on-line data access.
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