Path: utzoo!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: 32-bit LUCAS memory board
Message-ID: <5339@cbmvax.UUCP>
Date: 28 Nov 88 23:09:53 GMT
References: <6236@killer.DALLAS.TX.US>
Organization: Commodore Technology, West Chester, PA
Lines: 41

in article <6236@killer.DALLAS.TX.US>, aimania@killer.DALLAS.TX.US (Walter Rothe) says:

>  There are several observations/questions I had when a reviewed your
>  board as an aid to my own design work.

>  1) Dont you need a pullup resistor on AS00-.

If that's the 68000 AS*, there's one on the motherboard.

>  2) Why shut off AS going to the Amiga during a coprocessor cycle. Nothing
>     should respond. Wouldnt it eliminate alot of the grant circuitry
>     if you did not shut if off? Why is the flip flop needed to generate Z2-.

That really is necessary.  Memory boards and Amiga motherboard memory doesn't
respect the FC lines.  So allowing AS through during an FPU or interrupt
acknowledge cycle is going to cause trouble.

>  7) I dont understand how the 68K, in a standard Amiga, gets synced with
>     C1-C4 so that it doesnt put out an address strobe when C2 and C4 are
>     high. Does DTACK from accesses to standard memory come at a certain
>     time so that it gets synced with that. 

Access to CHIP RAM forces this synchronization.  CHIP RAM is really 
synchronous RAM, split between the 68000 and the CHIPs.  The CHIPs don't
handle asynchronous bus cycles like the 68000 can, so they always take
their half of the cycle at the same time.  DTACK* will be held off if
the 68000 comes in out of sync.  Once in sync, S2 (where AS* falls in
68000 terms) happens when C1 and C3 are both low.

> If there were only fast memory in a system, would it ever get synced?

It might not get synced.  This never happens, though, since you can't
operate without CHIP RAM, and the first RAM the system every touches
is CHIP, since that's the only RAM we know about at startup time.

> Walter Rothe at the UNIX(Tm) Connection, Dallas, Tx
> UUCP: {rutgers}!smu.killer.aimania
-- 
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