[B2-devel] MMU requirements in SheepShaver

[Jaime C. <jca...@gm...>]

Hi!

I'd like to know what are the MMU requirements to run Sheepshaver (in
order to port it to other PPC OSes like AmigaOS4, MorphOS or
AROS-PPC). Unfortunately I'm not used to work with MMU so I may be
wrong in some parts.

The idea is running SheepShaver like
Shapeshifter/BasiliskII/Fusion/iFusion did on AmigaOS, reserving a
part of the memory for the emulated Mac and using the cpu and memory
directly. As far as I understand:

1.-The PPC ROM is patched so it doesn't seek some bits of real mac
hardware. IIRC it can be placed in any part of the emulated mac memory
area so you don't have to use the MMU to remap it. The PPC ROM was
usually placed in a fixed address in real Macs until "newworld" macs
were released. Newworld macs load the "rom" in memory so it has to be
patched less. So I think you may want to use the MMU to mark it as
read-only but that's nearly all.

2.-The first (32?) kilobytes of memory are used by MacOS Classic to
store program variables or global settings, I can't remember. This
part can't be patched by our emulator as apps or MacOS are hardcoded
to write in that memory area. In AmigaOS3.x a special command was used
to keep those kilobytes of memory free for the emulator. Unfortunately
that would not be longer possible with AmigaOS4 and MorphOS (and it
isn't likely to change in future versions) and when an app tried to
write in that memory area I guess that an exception would be launched.
Unfortunately I don't know if reading would work... I hope it doesn't
but I doubt that memory area is read-protected since some parts of the
kernel and OS may read it. If that memory area can be read directly it
will probably rule out MorphOS as it doesn't provide a way to access
the MMU or advanced memory functions to user apps.

That would leave two options:
2.1.- When an exception/memory hit is launched because the host OS
marks that memory area as not readable/writable, try to catch it some
way (I wouldn't have very clear how to do it) and reproduce the
operation in a "safe" allocated memory area for the emulator. I don't
know if this could work.
2.2- Try to create an special "MMU context" for the emulator so it
remaps the first kilobytes of memory into another area we have
reserved. When the illegal instruction that turns back control of the
cpu to the host OS is executed, the original MMU context should be
restored so host OS functions aren't affected by this. I don't know if
the AmigaOS4 API allows this (amigaos3 mmu.library provided
functionality to do this). I guess this could be the right way to do
it.
2.3.- I think AmigaOS4 provides at least functions to mark the first
kilobytes of memory as not readable/writable so at least we may notice
when the data is being accessed. I still don't understand clearly how
could I "catch" this access and reproduce the changes in another
memory area.


So... is MMU used somewhere else in SheepShaver?


-- 
Saludos/Best Regards
Jaime Cagigal