Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!yale!mintaka!mit-eddie!rutgers!bpa!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Bridgeboards taking 2M of autoconfig space
Message-ID: <9825@cbmvax.commodore.com>
Date: 26 Feb 90 18:37:02 GMT
References: <1827@esunix.UUCP> <946@tardis.Tymnet.COM>
Reply-To: daveh@cbmvax.cbm.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 40

In article <946@tardis.Tymnet.COM> jms@tardis.Tymnet.COM (Joe Smith) writes:
>In article <1827@esunix.UUCP> blgardne@esunix.UUCP (Blaine Gardner) writes:
>>I don't own a Bridgeboard, but may need to get one in the future. I've
>>heard that they take 2M out of the autoconfig space.
>>Isn't the shared RAM only 128K? And doesn't the autoconfig protocol
>>provide for a minimum size of 64K? Unless there is something I'm missing
>>(quite likely), the Bridgeboards should only take 128K, not 2M.

Last I heard, BridgeBoards actually need 512K of memory.  They only have
128K of actual shared RAM, but it's mapped differently at 0k, 128k, and
256k offsets from the configuration base.  One mapping sends stuff straight
through, one adjusts words from big to small endian ordering, and one
mingles the bits of two words to make one longword (an operation called
"skiggling" in a really weird computer language I once heard about), the
idea being an assist to graphics translation from bitplanes to packed
pixels.  The final 128k chunk apparently has some I/O and control stuff in 
it.  

That's what they actually need; I have no idea if a BridgeCard actually
requests 512K of space or it it's really asking for 2 megs.  In the 
former case, you'd still be out of luck adding 8 megs of RAM, but 7 megs
should work OK.

>Too bad there isn't a way to make an Amiga memory board appear to the system
>as 3/4, 7/8, or 15/16 of its full size, so that the rest of the address
>space can be allocated to something else.  (DEC did that with their smaller
>PDP-11 systems for years.)

It would be nice, but there's no way autoconfig can handle that; the memory
grain a board responds to is pretty hardwired.  Some memory boards can be set
to subsets of their normal memory.  But most 8 meg boards only support 2, 4,
or 8 meg sizes (the ASDG design also does 6 megs).

>Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or jms@gemini.tymnet.com


-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
                    Too much of everything is just enough