Diskette Theory - IBM 6360 Product Support Manual

DISKETTE THEORY
The IBM Diskette is divided into 77
circular tracks numbered 00 through 76
(Figure 7-3). As the diskette turns,
data is magnetically recorded on these
tracks by the read/write head. Track
00 is the index or address track and
is divided into 26 sectors having 128
bytes of information each. This track
contains the information necessary to
identify the diskette and its
contents. The index track cannot be
used for data storage. Tracks 01
through 76 are data tracks and are
divided into 15 sectors for the Type
2D diskette." Track numbers 75 and 76
are reserved to take the place of any
primary tracks (01-74) that have
failed. The Type 1 can hold up to
284,000 bytes of usable information
and the Type 2D can hold up to 985,000
bytes of usable information.
Each sector of each data track starts
with its own address and contains 256
bytes of data. Each time the diskette
does a read or write operation, a com-
plete sector on the selected track is
either read into or written from the
main storage area.
The IBM Diskette is permanently con-
tained in an envelope for protection.
The inner surface of the envelope is
coated with a material which cleans
the diskette as it turns ins ide the
envelope.
Figure 7 -3. Diskette Tracks and Sec-
tors
There are six physical features on
Type 1 diskettes and seven on Type 2D.
(Figure 7-4):
1. Permanent Diskette Label. This
label is not removed. It is
available to the user for identi-
fying the diskette.
2. Temporary Label. This label is
adhesive and should be used for
temporary information such as the
last user, date, name of user,
etc.
3. Index Hole (Type 1 Diskette). The
outer circle identifies a hole in
the diskette 1 envelope; the inner
circle shows the index hole in the
diskette. When these two holes
are aligned while the diskette is
turning in the drive, the Dl Light
Emitting Diode (LED) will transmit
light to the Dl Photo Trans is tor
(PTX).
Chapter 7. Diskette Unit 7-3