At 11:39 PM 1/29/97, briggs@puma.macbsd.com wrote:
>> AFAIK, the only 68k Macs that do SCSI by DMA are the IIfx and the AV
>> machines (Allen, Scott?).
>
>This is correct.  The mac has "DMA" only in that all devices are memory
>mapped and nubus cards can access main memory directly.  The only macs
>that have DMA controllers that can drive devices independently are
>SCSI/DMA nubus cards (ATTO?, Jackhammer?, AWS95 PDS card?) and the IIfx
>(DMA used only by A/UX) and the AV machines--which really needs SCSI
>Manager 4.3 to make use of the DMA SCSI, I think.  I don't know how much
>the DMA engine has changed in the powermacs, but I doubt that it's
>changed too much from the AV machines.

I'm going by memory here and could be wrong.  What I remember is that the
650 and newer/higher models had DMA.  As you say you need SCSI Manager 4.3
in order to use it under MacOS (though as a single-tasking OS the
advantages are limited), and they never added DMA support to MacOS for the
IIfx.

The Quadra 900/950 had (still has I suppose:-) 2 SCSI busses, only one of
which was DMA.  (Probably much like the current 7500/7600/8500, though I
can't be positive.)  The PDS card that turns it into an AWS95 added 2 more,
again only one of which was DMA.

Signature failed Preliminary Design Review.
Feasibility of a new signature is currently being evaluated.
h.b.hotz@jpl.nasa.gov, or hbhotz@oxy.edu