[no subject]

Anyway... The FPI controls everything about the RAM in the IIgs.  It 
provides DRAM refresh and bit en/decoding.  It seems that the Data Bus from 
the Auxiliary Memory Slot and the Expansion Slots go directly though the FPI 
and aren't actually connected.  Since Apple designed the FPI to only account 
for 4 Megs of DMA access, 8 Megs doesn't seem possible.  Although addressing 
for 8 Megs is possible, DMA (Direct Memory Access) isn't.  Maybe it's due to 
the address multiplexing?  Someone?

No I'm no expert, so I hope someone will correct me if I'm wrong.  The IIgs 
Hardware Reference, as I said above, is really quite poorly written if you 
ask me.  Maybe it was designed for someone with more experience or knowledge 
then myself.  All I know is when I read it I hardly ever find the answers 
I'm looking for.  Is it me or do other people think the book is also poorly 
written?

So concerning DMA and 8 Megs:  It seems that the FPI is the issue.  Since 
it's soldered to the Motherboard there's no simple way to design something 
else to account for the DMA issues.  Then there's the question if all the 
necessary traces even run to the FPI to begin with.  I have been toying with 
the idea of possibly adding a CPU DIP plug to my version of an 8 Meg RAM 
card.  This would be a pass-through socket that would hopefully allow DMA to 
then be possible. Of course then without the FPI I would then need to 
account for DRAM refresh and address en/decoding.  There may be some other 
solution, but I've yet to find any others.  Any suggestions out there?  I 
had thought to clip on directly to the FPI, but a good PLCC clip seems to be 
quite expensive.  I'm not too sure I'd trust a PLCC socket turned upside 
down, but it may be worth trying.

Of course allowing full DMA access to all 8 Megs also raises the question of 
how GS/OS would handle it.  Does anyone think a custom driver would need to 
be written?  I can see cards like Apple II High Speed SCSI at least working, 
but I'm not too sure about others.  I think I also read somewhere that the 
TWGS would have issues with 8 Megs of DMAable RAM since they didn't follow 
Apple's hardware specs when they designed the board.  Whether this is true 
or not I'm not sure of though.

Can anyone answer my few questions?

Minor Update:  I now have a working 8 Meg RAM card.  The ROM circuit isn't 
fully finished but the RAM side of the card is working flawlessly.  As I 
mentioned above I'd also like to tinker with DMA support as well.  I've been 
thinking of producing a limited run of RAM only cards just to help raise 
some money to complete the project.  The cards would be $150 plus shipping. 
If you'd be interested in helping support the project contact me through the 
Web Site or email.  Of course users who purchase these cards now will 
receive a final version at a deeply discounted price.


-- 
Henry S. Courbis
www.GSE-Reactive.com
Legacy Hardware and Support - Come take a look at what we have to offer!