Ah, thanks, that was a great and well-detailed reply.
That clears a few things up a bit for me, and helped me realize something about what you said in an earlier post: according to you, the G5 processors implemented the UISA (User ISA) in a PowerPC-compliant way, meaning Book I of the POWER / PowerPC formal specifications, but not necessarily the rest (Books II, III and E).
Since the User ISA is 100% compatible and implemented between G5 and earlier PowerPC processors such as G3 and G4, doesn't that actually mean that Mac OS 9, which, being an OS, runs entirely on user space (excluding the nanokernel, which you suggested should be looked at separately) and, thus, is also 100% instruction-compatible with the G5 processor?
Assuming the answer to the previous question to be "Yes.", we are left with the nanokernel which, according to you, is normally used by OS 9, but is completely disregarded or even missing altogether in regards to Classic in Mac OS X (XNU). If I understood you correctly, the instructions you believe to be missing in the G5, which are on non-user-space, are present only in OS 9's nanokernel and are most likely emulated within XNU if XNU is running in a machine that lacks the native instruction.
Since Classic is a common program/process/task like any other that runs under Mac OS X, which in turn runs in user space, and since Classic delegates all the nanokernel calls to the XNU kernel instead, that means Classic itself does not and can not, by itself, emulate any missing instruction, but rely on XNU for all its nanokernel needs, including emulation of non-UISA-instructions, if needed. Classic is then unlike, for example, Rosetta on x86, which emulates the PPC UISA in an x86 UISA environment. And Classic IS like Rosetta in the sense that all that both can hope to achieve is stuck at UISA level (AKA user space).
With all this, I mean to conclude that, necessarily, Classic is not at all relevant to us here in terms of processor architecture compatibility or lack thereof for OS 9:
the XNU kernel is. Correct?
If correct, then my understanding is that to solve this processor architecture problem, an implementation on the OS 9 nanokernel is necessary to immitate whatever the XNU is doing in those cases. And the XNU only. Preferably Tiger's XNU as opposed to Leopard's, as it's the last XNU revision we know
for sure that had a motive to bother keeping OS 9 nanokernel instruction compatibility. Which is not to say that Mac OS X itself
necessarily didn't contain those very instructions themselves to be later emulated within the XNU on G5s.
(For emphasis, motherboard & other hardware drivers, though, are still entirely different subjects and dependencies than what I'm trying to get at here.)
All OS 9 nanokernel gurus out there and Naiw, please do correct me if I said some gibberish somewhere here.
I'm kinda walking blindfolded on some areas, but doing my best with it.
