Re: 5 Megs w/ DMA????

[JB3819@JB3819.SBC.COM]

In article <40p3ng$t4g@acme.freenet.columbus.oh.us>, 
<dalloff@freenet.columbus.oh.us> writes:
> 
> I have a 4Mb card (maybe cv tech?  can't tell you this second...) with a
> fully-populated Apple card piggy-backed on, in my ROM 01 GS, giving me a
> total of 5.25Mb.  I'm running a "stock" GS, with GS/OS 6.0.1 and a 20Mb
> Vulcan.
> 
> GS/OS seems to be recognizing all 5.25Mb.  Software works fine.  In 
fact,
> my GS runs for weeks without being shut down or crashing (wish I could 
say
> the same for the Amiga and the Macs at work...).
> 
> So just what is the problem with running more than 4Mb on a GS?
> 
> --Dave Althoff, Jr.

The problem isn't so much running a IIgs w/ more than 4MB, it is 
trying to do DMA to addresses higher than 4MB.  Some SCSI interface 
cards (RamFast, Apple HS SCSI, others?) use DMA (Direct Memory Access) 
to bypass the CPU and put the data into memory faster.  Due to the 
hardware design of the IIgs and some of the memory cards, attempting 
to do DMA to addresses over 4MB can lead to corrupted data.  The 
RamFast makers found a way to work around this problem and do DMA to 
all of RAM iff you don't have a TWGS (Applied Engineering didn't 
follow ALL the hardware rules when they made their IIgs accelerator). 
I don't think any other card does.  I believe (i.e., I could be 
wrong) the Apple High Speed SCSI card plays it safe and only uses DMA 
when the destination addresses are in the first 4MB.


JABouchard  "Poindexter"