toddpw@tybalt.caltech.edu (Todd P. Whitesel) (03/17/90)
NOSES@DBNINF5.BITNET (Achim Patzner) writes: >One minute please... Is there a slight chance that this card would work >without doin DMA too??? For all of you who don't remember: Everybody with >an 8 Mbyte RAM card in his system might become a bit frustrated now: As >far as I know is the IIGS's DMA hardware compatible to the earlier models >i.e. there will be only 16 bits of the destination addresse on the bus. >All cards (like those boards) that decode the address bus themselves might >have problems with this as they don't use the MMU's addressing signals. >Ok, we were warned when we bought them (I was, but only *after* having >bought it)... WRONG... anybody with a DMA compatible GS memory card will be able to use it WITH DMA. The GS has a register in it that tells the machine which 64K Bank the DMA access goes to, so you can DMA to the full 16 Megabyte address space (in theory) but you have to set up the DMA Bank Register before you do it. DMA compatibility problems on the GS started when Apple gave us 8 Megs of space dedicated to RAM but support for only four rows of RAM chips. Cards that get around the problem by decoding the bank address directly from the bus didn't work during DMA because of a bug in the FPI chip that caused the DMA bank register to not be put on the bus if the four supported rows of RAM chips weren't being accessed. Expander cards came up with a new fix: keep a copy of the DMA bank register on the card and use that. This works but gets messed over in the ROM 3 because the expansion RAM is now mapped to a different set of bank addresses. To avoid any problems, I'm going to buy a four row SIMM card. Four megs is more than enough for me, and with only four rows of chips I won't have any DMA problems. Todd Whitesel toddpw @ tybalt.caltech.edu