I might just shove a Blackbird in it to continue on the POWER legacy (even if it's not PowerPC).
Oh, rest assured, POWER9 is PowerPC. Like, literally. ...
POWER4 onwards are all pure PowerPC processors, and are no longer POWER. This includes the POWER9 present in the Blackbird. What happened is that the PowerPC ISA got, confusingly, renamed to POWER ISA. But it's all the very same architecture, with new instructions making their way in newer ISA versions, but without kicking out older ones (AFAIK).
Interesting! And IIRC, 970 did kick out a few instructions, which is part of why OS 9 won't run on them. X throws them to an exception handler. This is what I heard from Cameron, anyway...
Hm, I think the processor differences that prevent OS 9 from booting out-of-the-box in G5s come from things other than the actual ISA (the mnemonics and opcodes) per se, but I could be wrong, especially since I'm no expert on the subject.
Here we have a post that seems to show the probable actual reason, or one of the reasons.
As for the actual instructions in the ISA, if we look at
v3.0B (POWER9, according to
Raptor), between pages 1237 and 1255 of the PDF file (between pages 1219 and 1237 of the actual document), we have the list of OpCodes (instructions), mnemonics and the version of the ISA that introduced the instruction. We even see some POWER1 and POWER2 inheritance, as expected, since PowerPC was born out of merging POWER, Motorola 88k and Apple requests (related to the 68k?). Anyway, if we do some cross-referencing of that between the equivalent in v2.01 (G5) and earlier versions of the ISA, then we will be able to know for sure if anything got left out or not, instruction-wise.
Sounds like something to be done on a rainy weekend...