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3.1.2 Diskette Format
There are 77 tracks on the diskette surface; a
track is a circular path on the surface of the
diskette. The tracks are numbered 00 through
76. Track 00 is the outside track and track 76
is the inside track. Of the 77 tracks, only 74
tracks are data tracks. Track 00 is a label track
and tracks 75 and 76 are reserved as alternates
for tracks that become defective. Some diskettes
that are interchanged with other systems might
contain only 73 data tracks, track 74 is not used.
Each track is divided into either eight or 26 sec-
tors. The content of each sector is described in
the illustration on the facing page.
The data stored in one sector is called a record.
When the tracks are divided into eight sectors,
each record contains 512 bytes; when the tracks
are divided into 26 sectors, each record contains
128 bytes. Thus, diskettes that contain 74 data
tracks divided into eight sectors per track have
a capacity of 303,104 bytes of data; those divided
into 26 sectors have a capacity of 246,272 bytes
of data.
In some diagnostic programs, only one sec-
tor is written on a track. Each sector contains
4096 bytes of data.
Because the diskette is formatted into tracks
and sectors, each record on the diskette has a
definite address consisting of a track and sector
address. The address is recorded at the physical
location of the record on the diskette.
Diskettes that contain prerecorded record
addresses are called initialized diskettes. Initi-
alized diskettes contain an I D field for each
record.
Each record on the diskette consists of two parts;
the first part contains identification information,
and the second part contains data.
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26 Sector Diskette
3-4
Track 76
Track 00
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