Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cbmvax!valentin
From: valentin@cbmvax.UUCP (Valentin Pepelea)
Newsgroups: comp.sys.amiga.tech
Subject: Re: DMA in VM
Message-ID: <8909@cbmvax.UUCP>
Date: 9 Dec 89 10:47:34 GMT
References: <14059@grebyn.com> <838@tardis.Tymnet.COM> <14068@grebyn.com> <8800@cbmvax.UUCP> <9383@microsoft.UUCP>
Reply-To: valentin@cbmvax.UUCP (Valentin Pepelea)
Organization: Commodore Technology, West Chester, PA
Lines: 27

In article <9383@microsoft.UUCP> w-edwinh@microsoft.UUCP (Edwin Hoogerbeets) writes:
>In article <8800@cbmvax.UUCP> valentin@cbmvax.UUCP (Valentin Pepelea) writes:
>>The 68851 MMU resides between the main processor and memory. If a DMA device
>>wanted to access the memory through the MMU, it would have to be sitting on the
>>same side as the processor. Obviously impossible.
>
>Which side of the MMU are the custom chips? 

There is no MMU in the Amiga 500 & 1000, so obviously on the A2620 and A2630
accelerators, the MMU filters only the CPU's memory accesses.

>If they are on the same side as the processor, does this mean it will
>obviate the distinction between chip and fast RAM, since the Meg or so
>that these chips could access could be remapped to be anything? That
>would be neat.

Actually, that would be disastrous. Even if a bright engineer could figure out
how to do that, the overhead introduced by the MMU would render things very
slow. More like frozen. If you want more CHIP ram, it is easier to make that
chip access more ram rather than building or externally attaching an MMU to it.

Valentin
-- 
The Goddess of democracy? "The tyrants     Name:    Valentin Pepelea
may distroy a statue,  but they cannot     Phone:   (215) 431-9327
kill a god."                               UseNet:  cbmvax!valentin@uunet.uu.net
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