magnetic tape

storage (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 "roundtape".

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

Exabyte.

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 ("blockedrecords"). Finding a certain block on the tape generally

involved reading sequentially from the beginning, in contrast

to magnetic disks. Tape is not suitable for randomaccess. 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

DLZ1 (DLT).

See also cut a tape, flap, Group Code Recording,

spool, macrotape, microtape, Non Return to ZeroInverted, Phase Encoded.

(1997-04-05)