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3-2 Theory of Operation
The makeup of the ID and DATA fields is similar. Both fields begin with a series of synchro-
nization bytes. These bytes allow the decoder circuitry of the controller time to synchronize
itself with the data on the disc. Following the synchronizing bytes, is the address mark byte
which indicates that the beginning of an ID or DATA field has been located. The first bit of
an address mark is the opposite polarity of the last bit of the previous synchronizing byte.
This feature simplifies detection of address marks.
A series of information bytes follows the address mark. In an ID field, these bytes indicate
the logical cylinder, head and sector address. In a DATA field, these bytes are the data being
stored in the sector.
At the end of each field are two cyclic redundance check (CRC) bytes. This check word (16
bits long) allows detection of most errors that occur in the data storage and recovery of
information from a disc.
There are two gaps following each field on a track. The gaps allow for variations in disc
rotational speed, index detector alignment variations and time for the hardware to prepare
for the next field.
The logical sectors are numbered consecutively. However, the sectors (see Figure 3-2) may
occur in any physical order around the track. This allows the sectors to be staggered to
optimize system performance (interleaving).
The outermost track on the disc is track 0 and the innermost track is track 34. Each track has
a physical address as described preViously. There is also a logical track address associated
with each good track. The logical track address is written in the ID field of each sector on the
track. If a flexible disc has no bad tracks, the logical track has the same address as the
physical track.
The recording head (see Figure 3-3) is moved in and out by a stepper motor assembly.
Write current passes through the head coil to selectively magnetize the portions of the disc.
To read back data, the magnetized material is passed under the head, thereby indUcing read
current into the head coil.
35 concentric tracks
on each side
(2 are reserved)
Addressable sectors:
16 sectors
33 tracks
Sector
numbers
(16 total)
- - x
~~-
x 2 sides = 1056 sectors
track
side
Figure
3-2:
Media Sector and Track Structure
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