Doorst: Re: PC-card - DMA/SBHE
Date: 2001-04-16 12:27:34


> I found a 1984 16-bit 4 MB memory card and looked for pins connected to 
> this line using a beeper. I only found inputs. 

After sending previous mail it occured to me to check the datalines. The 
SBHE-line enables a 245 which buffers D8..15. The other side of the 245 
goes to the datapins of one half of the DRAMs. Another 245 buffers the 
other half and is enabled by A0. I didn't find any cross-over of datalines 
which would enable one to read/write D8..15 using D0..7.

So IMHO DMA _must_ be an 16 bits process. Then what about A0 becoming "1"? 
In that case the D8..15 are read/written a second time.

The only point of light I see for the moment is that an AT can DMA to an 
8-bit card. So on the motherboard there must exist a mechanism that deals 
with the 16-bit -> 8-bit transformation during a DMA. 

Hmmm, what about keeping A0 towards the card a "0" when the 8237 outputs a 
"1" and copying the upper into/from our own upper-byte register?

It sure helps to talk with friends about your problem :)

Groetjes, Ruud

Groetjes, Ruud

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