Re: 5 Megs w/ DMA????

[sbehrens@contech.demon.co.uk (Soenke Behrens)]

dalloff@freenet.columbus.oh.us (Dave Althoff) wrote:

>Brian D. Uhreen (bduhreen@freenet.calgary.ab.ca) wrote:

>: Ok perhps you can help me with this question..I have a FULL cv tech 
>: board..(4 megs) with the 1 meg apple board piggy backed.  I also use the 
>: Apple high speed scsi.  If I enable the DMA on teh scsi card ...well.. 
>: things don't work well.  Anyway..is there something I can do about this 
>: short of pulling the apple board?  And f all I can do is pull the apple 
>: board is it woth the DMA to go with out the extra meg of ram??

>Could someone explain to me just what the problem is?

DMA is the problem. Traditionally, you can only use 4MB for DMA, because
the DMA bank latch is only valid for the lower 4MB of expansion RAM (so,
really, it is 4.25MB you can use in a ROM1, and 5.125MB in a ROM3). There
are two ways around this for a DMA-using card:

1.) Know about the limit and don't DMA into the upper 4MB, DMA into a
buffer in the lower 4MB, then copy, or use plain PIO for the upper 4MB
2.) Use an undocumented RDY feature to halt the CPU in such a way that the
card can "play CPU" and put the full address on the bus, then write the
data. This trick doesn't work with a TWGS (does work with a ZIP), and the
only card that uses it so far is the RamFAST SCSI.

>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...).

Your Vulcan doesn't do DMA. Even if you were to put in a TurboIDE (which
does DMA) to speed up your vulcan, you wouldn't have a problem, because the
TIDE knows about the 4MB limit and bypasses it by method 1, above.

>So just what is the problem with running more than 4Mb on a GS?

If you have an Apple HS SCSI card or a Vision Eyes Digitizer (that does
DMA, doesn't it? Or was it some other card?), then you will have a problem:
These cards do DMA, but don't take the traditional 4MB limit into account,
so they will just crash your machine if you have more than 4MB of expansion
RAM.

Yours
Soenke




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