Date   : Mon, 03 Aug 1987 01:44:00 PM
From   : Elliot Mednick
Subject: XEROX 820 DD Package revisited

Last week, I posted a query concerning a package by Software Publishers,
Inc. which converts single density disks on my Xerox 820 to double density.
Thanks to Mitch Mlinar for his response.

I've been experimenting with the problem and have come up with some interesting
results which I hope somebody can explain.

The experiment is to take a disk, format it with one of several combinations of
DD format and skew, copy to it ADV02A.LBR, extract ADV.CQM using LU, and USQ
*.?Q?.

In the first trial, I used 9 sectors/track at 1024 byte sectors.  During the
USQ, two sectors came up bad.  Repeating the experiment (including formating
and verifying the format), the SAME two sectors came up bad.  When I tried
changing the DD format to 26 sectors/track at 256 byte sectors, two DIFFERENT
sectors came up bad, but at about the same time into the USQing.  Again, this
was repeatable.  But, when I tried both 50 and 52 sectors/track at 128 bytes
(the magic number), there were NO errors.

It sounds like I have a software problem while writing a certain pattern to the
disk or while writing a certain combination of virtual 128 byte blocks.  The
problem is not with LU since I have been able to blow up sectors while
downloading through KERMIT and other *seemingly* random times.  But something
that LU does cause the problem to show up.  Does anyone know what LU does twice
to extract ADV.CQM that it otherwise doesn't do?  I'll suppy any information
about sectors numbers if necessary.  The bad sectors show up about 1/3 and 2/3
into the USQing of ADV.CQM.

My next step is to disassemble to BIOS and try to put in moustraps.  If anyone
can give me any ideas, I'd be very appreciative.

-Elliot Mednick
arpa:  elliot@s34.prime.com
uucp: ...mit-eddie!primerd!s34!elliot