NuBus Master Cards - recipe for trouble

[Paul Campbell]

In article <47191 at apple.Apple.COM> lantz at Apple.COM (Bob Lantz) writes:
>
>Although A/UX uses the same data structures as MacOS (indeed, we have 32-bit
>QuickDraw, etc..) A/UX (and other virtual memory systems for the Mac) pose
>some problems for NuBus cards which expect to have direct access to the Mac
>address space.  In MacOS your physical addresses and virtual addresses are
>generally the same (with some exceptions,)  whereas in A/UX they are not.  

There is only one accelerated video card out there that I know of that does
bus accesses to main Mac memory and potentially has problems with VM/AUX -
that's the Apple 8*24GC.

Other cards DO have problems because A/UX DOES have different data structures
from the MacOS for video cards - A/UX splits video drivers half between
the kernel (for vertical retrace interrupt handling to load palettes
and to move the cursor) and user space (for application drawing etc).

I'm not saying it can't be done (it's certainly much easier for us than
for Apple :-), the real problem is putting a lot of work into doing this
stuff for a pretty small market and running the risk of having Apple
change the interface out from under us again (like 1.0->2.0, I think
that after 2.0 this will become less of a problem).

	Paul Campbell
	Accelerator Architect and A/UX Hacker
	SuperMac Technologies
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