Path: utzoo!utgpu!attcan!uunet!seismo!sundc!pitstop!sun!oliveb!amiga!neil
From: neil@amiga.UUCP (Neil Katin)
Newsgroups: comp.sys.amiga
Subject: Re: Request to Commodore (Bad Blocks)
Keywords: trackdisk floppy format
Message-ID: <2979@amiga.UUCP>
Date: 30 Sep 88 21:35:40 GMT
References: <8891@cup.portal.com> <5660016@hpcvca.HP.COM> <4774@cbmvax.UUCP> <2666@sugar.uu.net>
Reply-To: neil@spam.UUCP (Neil Katin)
Organization: Commodore-Amiga Inc, Los Gatos CA
Lines: 32

<I tuned in late to this discussion; please don't flame me too bad>
<if I missed something that was said before>

As I understand the proposal, we want to change the way the trackdisk
writes to the disk: to sync writes with the index mark.  This is
supposed to help recover the disk if there is a write error.  Reading
the disk would be unchanged.

What escapes me is why this would help...  The trackdisk uses
a level 6 interrupt to synchronize with the index mark.  Because
it is an interrupt (and therefore s/w gets involved) anything that
stops or slows down interrupts will "shift" the start of the track.
This can include programs that call Disable() or things that starve
the processor of cycles (like a hard-disk doing DMA, or 4 bit planes of
high res, or the co-processor getting all the cycles, or the blitter
with the nasty bit set, etc).

Therefore your recovery s/w needs to be able to deal with tracks that don't
start at the index mark.

I have a different proposal.  First of all, what's a bad, but recoverable,
disk (in the current format, at least)?  It's one that has some good
sectors, and some bad ones.  It also probably has some sectors that
that were left from the last time the disk was written.  Currently
there is no way to tell if the sectors are from the "new" track,
or the "old" track.  One way to cure this is to add a two bit "generation"
number to each sector.  That way you can uniquely identify which
sector is new (or old), and you don't need to wait for the extra half
rotation for index to roll around.

	Neil Katin
	new mail address: {half the world}!sun!katin