Date   : Tue, 21 Aug 2001 08:55:13 -0700 (PDT)
From   : Thomas Harte <t.harte@...>
Subject: WD1770, disc drive & sector dump file questions

I know this doesn't seem to be particularly within anybody on this list's
field of knowledge, but I wondered if anybody could answer these very simple
questions :

- what is the effect of resetting the WD1770? Assuming that bit is active
high, the electron format utility has the odd behaviour of, after formatting
is complete, writing a 1 to the 'drive control reset' bit of the BBC
B+/Electron specific drive control register, then performing a seek to track
0 having set the track register of the WD1770 to 0 also and expecting that
to work. So my guess would be that doing a reset either has the net effect
of a 'restore' command, or a 'read address' where the address is then filled
into the WD1770 registers.

Of course, the effect of resetting the wd1770 isn't mentioned on the
datasheet or online as far as I can see. That'd make everything far too
easy!

- what is the effect of reading drive interface lines when both drives are
selected? I know it isn't a normal thing to do, but enabling both drives at
once is the only way to step both heads at once, so I guess some software
might do it. But are the lines wired to produce a logical or, or a logical
and, or what?

- the smallest question of all relates to those horrid sector dump image
files that are so prolific. I've noticed that a lot of them aren't full
length - i.e. they may only be 32kb in size if the files contained on them
don't use any sectors above that limit. Which is an effective argument
against interleaving since it can as much as double the size of ADFS
floppies stored in that way, and will raise the size of DFS floppies which
don't have exactly the same amount of data on both sides, but my question is
: is there an accepted set of rules for this image chopping? Am I only meant
to round to the next track or the next sector or something like that if I
want to maintain compatibility with all the old tools?

-Thomas





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