Sector Skewing
Skewing is the rearrangement of sectors on a disc, so that by the time the
computer has read and processed one sector, the next will be in the right
position for the disc controller to read. Otherwise the poor controller has
to wait for the disc to make a full revolution before the right sector appears.
There are two types of skewing - software and hardware.
Hardware skewing
In this system, when the disc is formatted the sectors are laid out in the
required order - for example 1, 4, 7, 2, 5, 8, 3, 6, 9. The retrieval of the
correct sector is handled transparently by the disc controller and no special code
is needed to handle such discs.
Software skewing
In this system, the disc is formatted with the sectors in numeric order -
for example 1, 2, 3, 4, 5, 6, 7, 8, 9. However, the software which reads/writes the
disc has a translation table which it uses whenever supplying sector numbers to the
controller - this might read 1, 3, 5, 7, 9, 2, 4, 6, 8 - so if a program writes to
what it thinks is the sixth sector of a track, this is translated to sector number 2.
The problem with software skewing comes when another computer wants to read the
discs created by the first. Unless the contents of the translation table are available,
the information on the tracks will appear to be hopelessly jumbled up.
Under CP/M 1, the skewing is fixed for its 8" discs. The table reads:
1,7,13,19,25,5,11,17,23,3,9,15,21,2,8,14,20,26,6,12,18,24,4,10,16,22
Under CP/M 2 and later, software skewing is handled by the SECTRAN
system call. SECTRAN requires the address of a translation table; see the description of the Disk Parameter Header to
find the address of this table.
