|(storage)||magnetic tape - (Or "magtape", "tape" - paper tape is now
obsolete) A data storage medium consisting of a magnetisable
oxide coating on a thin plastic strip, commonly used for
backup and archiving.|
Early industry-standard magnetic tape was half an inch wide
and wound on removable reels 10.5 inches in diameter.
Different lengths were available with 2400 feet and 4800 feet
being common. DECtape was a variation on this "round tape".
In modern magnetic tape systems the reels are much smaller and
are fixed inside a cartridge to protect the tape and for
ease of handling ("square tape" - though it's really
rectangular). Cartridge formats include QIC, DAT, and
Tape is read and written on a tape drive (or "deck") which
winds the tape from one reel to the other causing it to move
past a read/write head. Early tape had seven parallel tracks
of data along the length of the tape allowing six bit
characters plus parity written across the tape. A typical
recording density was 556 characters per inch. The tape had
reflective marks near its end which signaled beginning of tape
(BOT) and end of tape (EOT) to the hardware.
Data is written to tape in blocks with inter-block gaps
between them. Each block is typically written in a single
operation with the tape running continuously during the write.
The larger the block the larger the data buffer required in
order to supply or receive the data written to or read from
the tape. The smaller the block the more tape is wasted as
inter-block gaps. Several logical records may be combined
into one physical block to reduce wastage ("blocked records"). Finding a certain block on the tape generally
involved reading sequentially from the beginning, in contrast
to magnetic disks. Tape is not suitable for random access. The exception to this is that some systems allow
tape marks to be written which can be detected while winding
the tape forward or rewinding it at high speed. These are
typically used to separate logical files on a tape.
Most tape drives now include some kind of data compression.
There are several algorithms which provide similar results:
LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and
See also cut a tape, flap, Group Code Recording,
spool, macrotape, microtape, Non Return to Zero Inverted, Phase Encoded.
, cassette tape
, mag tape
, magnetic stripe
, memory device
, storage device