Online Computer Terms Dictionary - M

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 "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 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 ("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 DLZ1 (DLT).

See also cut a tape, flap, Group Code Recording, spool, macrotape, microtape, Non Return to Zero Inverted, Phase Encoded.

(1997-04-05)