Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!apple!oliveb!amiga!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: LUCAS board fix (long)(technical)
Message-ID: <6232@cbmvax.UUCP>
Date: 13 Mar 89 17:58:06 GMT
References: <3291@pixar.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 32

in article <3291@pixar.UUCP>, gbuce@pixar.UUCP (George Buce) says:
> Keywords: LUCAS 68020 accelerator u9 swapping fix
> Summary: no more U9 swapping

> Since we know that the first DTACK* comes from within the Amiga during the 
> call to ROM directly after reset, and the Amiga specs state that you aren't
> allowed to return DTACK* before the beginning of S4, we know that the DTACK
> signal will fall somewhere during the S4 state.  

No, actually, we don't KNOW that (I'm not sure if it's true for an unexpanded
A1000, but it's not true for an unexpanded A500 or A2000).  The bus master
(the 68020, in this case) can't ACT on DTACK* before the falling edge of
S4, which is where DTACK* should be sampled by a device like LUCAS.  But 
DTACK* can really fall at any time, legally.  In fact, you almost certainly
get a glitch on DTACK* at around S3 when asserting OVR* from the expansion
bus to drive your own DTACK*.  That's nothing to worry about, though, based
on the definition (from the 68000 spec) that DTACK* isn't sampled until
that S4 edge.

With that said, your fix probably has a good chance of working on an A1000
with no memory added, based on the way DTACK* is actually generated by the
1000.  With external memory added, there's no guarantee you'll stay synced,
since a memory board or other expansion device can legally add any number
of wait states, which looks like it would be bad if you're really trying
to stay synched to a particular part of the Agnus cycle.

> George (8{>

-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
              Amiga -- It's not just a job, it's an obsession