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6.4 / MAGNETIC TAPE 211

(0.5 inch).Tapes used to be packaged as open reels that have to be threaded through
a second spindle for use.Today, virtually all tapes are housed in cartridges.
Data on the tape are structured as a number of parallel tracks running
lengthwise. Earlier tape systems typically used nine tracks.This made it possible to
store data one byte at a time, with an additional parity bit as the ninth track. This
was followed by tape systems using 18 or 36 tracks, corresponding to a digital word
or double word.The recording of data in this form is referred to as parallel record-
ing. Most modern systems instead use serial recording, in which data are laid out
as a sequence of bits along each track, as is done with magnetic disks. As with the
disk, data are read and written in contiguous blocks, called physical records, on a
tape. Blocks on the tape are separated by gaps referred to as interrecord gaps. As
with the disk, the tape is formatted to assist in locating physical records.
The typical recording technique used in serial tapes is referred to as
serpentine recording. In this technique, when data are being recorded, the first set
of bits is recorded along the whole length of the tape. When the end of the tape is
reached, the heads are repositioned to record a new track, and the tape is again
recorded on its whole length, this time in the opposite direction. That process con-
tinues, back and forth, until the tape is full (Figure 6.14a). To increase speed, the

Track 2

Track 1

Track 0

Direction of
Bottom read—write
edge of tape
(a) Serpentine reading and writing

Track 3 4 8 12 16 20

Track 2 3 7 11 15 19

Track 1 2 6 10 14 18

Track 0 1 5 9 13 17

Direction of
tape motion
(b) Block layout for system that reads—writes four tracks simultaneously
Figure 6.14 Typical Magnetic Tape Features