mdj wrote:
On Mar 13, 4:55 am, "Michael J. Mahon" <mjma...@aol.com> wrote:Add an accelerator to the host and sky's the limit. You cannot use an accelerator with a DMA device. Applicard works with any accelerator on any version of the A2.For me, this is a huge advantage. It's too bad that most accelerators were not implemented with DMA compatibility (though I know it would have complicated them).For Zip Chips, it always struck me as odd, since the cache writes through via a buffer. The DMA setup time should have been adequate to dump the write buffer. All it needed to do then was flag the cache as dirty.
I think the problem is that DMA is the only case of something besides the processor writing to memory. This means that if DMA can be ignored, the cache logic only has to pay attention to addresses generated by the processor, not addresses that appear on the bus. Since the processor is synchronous with the cache logic, this side is easy. Snooping bus addresses would have been a completely new address datapath running between the two clock domains. Since prior to the introduction of the High-Speed SCSI card, the penetration of DMA into the Apple II world was extremely low, it was an understandable decision to forego DMA compatibility to achieve a significant simplification. Then there's the problem that DMA is not directly signaled to the pins of the 6502 socket... ;-) -michael ******** Note new website URL ******** NadaNet and AppleCrate II for Apple II parallel computing! Home page: http://home.comcast.net/~mjmahon/ "The wastebasket is our most important design tool--and it's seriously underused."