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Theory of Operation 2-11
The makeup of the ID and data fields is similar. Both fields start with a series of sync-up
bytes. These bytes end with a long string of identical bits. During a read, the bit string allows
the controller's decoder circuitry time to synchronize itself with the data on the disc. Next
comes an address mark byte, which indicates that the beginning of an ID or data field has
been found. The data stored in this byte indicates which type of field
it
is part of. In order
that no other byte can be mistaken for an address mark, the address mark byte contains an
abnormal pattern of clock transitions. The first bit of an address mark is the opposite type
from the last bit of a sync-up field. This feature simplifies detection of address marks.
Following the address marks comes a series of information bytes. In an ID field, these bytes
indicate the logical track, 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 Redundancy Check (CRC) bytes. These bytes allow
the detection of most errors that occur in the storage and recovery of information from the
disc.
There are gaps between each field on a track. The gaps allow for variations in disc rotational
speed. The sectors are logically numbered consecutively. However, the sectors may occur in
any physical order around the track. This allows the sectors to be staggered to optimize
system performance.
t I
Gap
1
Sector
I
Ga p
I \ \ I
Sector
I
Gap
I
~
~~/~------~~-~-~----
I
- - - _
I
- - - - __ _
I
- - - ____ _
I
_ _ _
I
-
_ _
,
---
. . - - - - - - - - - 1 0 Field
------...-1
..... - - - - - - - - - Data Field
------l~
Figure 2-3. Track Format
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