Western Digital WD1007A-WAH User Manual page 14

Sector Skewing
Sector skewing is a method of formatting a drive by which sector numbers are rotated in
the interleave table each time a new head on a cylinder is formatted Selecting a skew of
2 causes the first sector after index on Head 0 to be identified as Sector 1. The sector
identified as Sector 1 on Head 1 is the third physical sector from index as shown in the
following example:
Example: Ten sectors per track with a skew of 2 and 1:1 interleave
Head Sector Numbers
0 1 2
1 9 10
2 7 8
This formatting procedure allows the controller to maintain 1:1 interleave when reading
across the head boundary. This becomes critical when the number of sectors per track
increases and the time allowed for completion of overhead functions decreases. By
changing the sector numbers the controller has enough time to complete needed tasks
and be ready to read the ID field of the first sector on the next head.
The minimum sector skew factor for proper performance of the WDl007A is 2. Different
applications may require different skew factors to optimize performance.
Spare Sector
WDFMT 2.10 also provides the ability to format a spare sector on the track. This spare
sector is identified as Sector 0, making it invisible to the system BIOS which expects
sector numbers starting at 1. This sector is always formatted as the last physical sector
on the track. The surface analysis portion of WDFMT uses the spare sector if an error is
encountered with any sector on a track. The program reformats the track, numbering the
bad sector as zero and shifting the following sectors one to the right. If more than one
sector is bad, the entire track is marked as bad when reformatted.
This feature is useful since many system BIOS ROMs support ESDI drives that have 34
sectors per track The spare sector option causes the drive to look like it has only 34
sectors while providing a spare sector for bad sector reassignment. Software drivers can
also make it possible to utilize the spare sector for storing custom data.
3 4 5 6
1 2 3 4
9 10 1 2
7 8 9 10
5 6 7 8
3 4 5 6