On Thu, 1 Sep 2022, Sellam Abraham via cctalk wrote:
This is because, like Apple ][ drives, they used GCR encoding and looked
for sync bytes on the disk rather than implementing additional hardware to
look for the index hole.
Sellam
Well, it's not really just the GCR that does that.
On the IBM/WD style FM/MFM formats (calling them "IBM format" creates lots
of confusion for people who don't know about anything before the IBM
PC/5150, and think that "IBM format" means "PC format"), the index hole is
used to mark where the track starts, and is necessary for formatting.
Once the disk is formatted, reading and writing can be done without it.
IF'n you were to format a disk without it (indexing off of the spindle, or
custom formatting software that starts a track wherever it happens to be,
such as the AppleII (which also used GCR instead of FM/MFM)), then the
disk would still be mostly usable.
The index hole/pulse is also used by some drives, to tell whether the disk
is turning, and or has turned too many times without a successful read, .
. .
And, the NEC 765 type controllers can't read ANYTHING for a short time
after the index pulse ("flash blindness") SOME computers using WD MFM
controllers start the first sector too soon after the index pulse for the
765 controllers to handle. Masking the index pulse (in the cable, board,
or write-protect tab over the index hole access hole) can sometimes work
around that.
--
Grumpy Ol' Fred ci...@xenosoft.com
