Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!bagate!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Bridgeboard and Fast Memory?
Message-ID: <20260@cbmvax.commodore.com>
Date: 2 Apr 91 15:14:17 GMT
References: <19620015@hp-lsd.COS.HP.COM>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 36

In article <19620015@hp-lsd.COS.HP.COM> davem@hp-lsd.COS.HP.COM (Dave K. Martin) writes:

>Why is 6-meg of fast ram the ideal configuration for an A-2000 with a
>bridgeboard?  

It's the most autoconfig memory you can easily use on an A2000 and still have
a bridgeboard installed.

>Does it have to do with address conflicts on the Zorro buss and the shared 
>memory on the bridgeboard?
							^^^^
That's "bus"; "Buss" is a fuse company, "buss" is what our grandparents did on
dates, like "spooning"...

The Zorro II implementation on the A2000 supports a total of 8.4MB of expansion
space, divided up into an 8MB chunk and a 488K chunk.  BridgeBoards require
512K of space, leaving 7.5MB of space left in the 8MB chunk.  6MB is the 
closest approximation to 7.5MB that makes sense to most designers.  One of the
problems is that autoconfiguration units can only be in powers of 2, between
64K and 8MB.  So to get 4MB or 8MB, you only need one autoconfig unit.  To get
6MB, you need two, and to get 7.5MB, you would need four.  Each new autoconfig
unit is extra hardware complexity, and as far as memory goes, each new one 
would get you less and less additional benefit.

>What happens if I get another 2-megs of fast ram?  

You run out of configuration space.  The last card in your system doesn't get
configured.

>davem


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
      "That's me in the corner, that's me in the spotlight" -R.E.M.