FC0-FC2 680x0 Bus Signals Needed? — #18

Post #18 by trag

Source Forum	68kMLA
Category	Troubleshooting
Post Date	Thu, 18 Aug 2011 - 23:53
Original URL	https://68kmla.org/bb/threads/fc0-fc2-680x0-bus-signals-needed.23694/

Post

. (In the case of the Apple II, hey, make the first ever 512MB DDR "Slinky" RAM card!) But hey, whatever works.

Heh, heh. I keep seeing those Apple II high speed SCSI cards going for $150+ and thinking, "How hard could that be to clone?"

I would strongly recommend looking at the work the MESS developers have done in documenting the Mac hardware:
http://mess.redump.net/mess:driver_info:mac_technical_notes

helpful diagrams of how the physical address space is mapped into the 24 and 32 bit logical address spaces depending on how the machine is configured. It's all done with the MMU.

There is a "RAM BASE" global variable, but I'm skeptical that just changing it to point to a bank of memory "somewhere high" would do what you want.

I'm pretty sure what "Mode32" mostly does is fix some bad/missing MMU maps so the early machines operate properly in 32 bit mode. If that's the case then it may well be possible to make a custom Mode32 that can operate as an OS patch with stock SE/30 ROMs, although it may almost be simpler to make a custom ROM upgrade based on the IIsi/ci 32-bit clean ROMs that natively supports discovering and mapping "high" memory.

Or I could be *completely* wrong about everything. Your mileage may vary.

Thank you for the pointers. Those are all avenues which will need exploring. Happily, I don't need to explore them right now. At the moment, all I need to do is correctly identify which hardware signals must reach the FPGA in order to be able to do what I want.

At worst, even if I can't map any of the extra memory as RAM, it sounds like dbraun (?) has figured out the basics of how to turn it into a RAM disk. So at worst, I could turn it into a big RAM disk and use it as swap space for virtual memory.