Posted by Bob on October 07, 19101 at 11:16:55:
In Reply to: Hard Disk sector numbering posted by Mark Gilbert on September 24, 19101 at 20:40:17:
: Why does a Hard drive skip sector numbers? Thanks
I think you are referring to INTERLEAVING, where
the sectors are numbered in a pattern that gives
the slow electronics in some older controllers
time to process the last sector that has been
read before the next sector in sequence is under
the head. If the sectors were not rearranged, the
next sector would have gone past the head before
the circuitry is ready for it, forcing a wait
for almost a full revolution of the platter
before it comes around again. A common interleave
was 3:1, where every third sector came next:
0 6 12 1 7 13 2 8 14 3 9 15 4 10 16 5 11
It took 3 revolutions of the platter to read the
entire track, but that beats 17 revolutions!
A faster controller might be able to do 2:1 as in
0 9 1 10 2 11 3 12 4 13 5 14 6 15 7 16 8
(Both examples are for 17 sectors per track.)
Eventually controllers got fast enough to keep
up with 1:1 "interleaving", where the sectors are
not rearranged at all. Note that another practice
called skewing would offset the location of
sector zero on adjacent tracks and between
platters so that switching from head to head
or track to track didn't let the sector go by
before it could be read.
Today's drives have the controllers on board and
use variable numbers of sectors per track from
the inner to the outer tracks, and all of this
is hidden from the user and the BIOS by the on-
board controller.