Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mit-eddie!rutgers!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Hey, Commodore Sales:  Amiga 3000 Warning
Message-ID: <3660@cbmvax.UUCP>
Date: 20 Apr 88 17:47:04 GMT
References: <1359@hubcap.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 34

in article <1359@hubcap.UUCP>, rchampe@hubcap.UUCP (Richard Champeaux) says:
> Summary: Is it as easy as you claim?

>      You're right, if the 68030 is anything like the 68020, it is capable of
> dynamic memory port sizing, and would have no problem addressing a 16-bit
> card.  However, if that card has DMA on it connected to some I/O device,
> then it would need to be able to address the 32-bit memory that is most likely
> to exist in an A3000.  Getting a card that is connected only to D0-D15 of the
> address bus to access the odd addressed words connected to D16-D31 would
> require extra hardware on each of the 32-bit memory cards that would allow
> word swapping.  

It's not all that complicated.  We do it on the A2620 card.  When a 16 bit
DMA occurs to 32 bit memory, odd words go to one half of the data bus, 
even words go to the other half.  It required exactly two extra 8 bit buffer
chips and one additional buffer control line.  

It might, however, get more complex when such a setup is running on an
expansion bus.  On the A2620, we have the advantage that all transfers are
happening through our controlled buffers.  On a bus that shares 16 and 32
bit devices, this is no longer the case.  A 16 bit device may want to
address a 32 bit device.  That necessarily places the burden of word 
shifting on the 32 bit device instead of the motherboard system, as long
as the shared bus is actually sharing data lines.  So there's some
additional logic required, probably the same buffers I use on the A2620,
but they'd be required on every 32 bit card unless a better scheme can
be discovered.

> Rich Champeaux
> Clemson University
-- 
Dave Haynie  "The B2000 Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"