[OpenBIOS] Apple's BootX

[BALATON Zoltan]

On Sat, 27 Jan 2018, Jd Lyons wrote:
> Yes, I think a custom kernel maybe what’s needed, there is some magic 
> that qemu-ppc is not doing. A custom kernel can boot on the 7448, as I 
> have already confirmed. I was hoping to avoid that, but until someone 
> that is familiar with how qemu-ppc boot OS X, I’ll just have to try the 
> custom kernel route.
>
> For the 7448, I used the kernel linked in these tread:
>
> https://forums.macrumors.com/threads/os-x-tiger-on-a-603-604-cpu.1908276/
>
> If we can figure out what changes were made that allowed qemu-ppc to 
> boot with it, we should be able to figure out how to get the 
> 7447a/7450/7455 to boot with it.

I'm just missing the point doing this. If it already works with the CPU 
type called G4 in QEMU (which seems to be an alias to 7400_v2.9) what 
would it bring to make it also boot with all these other CPU types that 
seem to be newer versions of the same G4 but were not normally found in 
real Macs?

I don't know these CPUs too well but if you're hoping to achieve faster 
emulation by emulating chips that were faster in hardware I'm not sure it 
works that way. Emulation is different than hardware so what's faster in 
hardware may actually be slower in emulation if it's more complex and thus 
there're more things to emulate versus one that's simple to emulate (that 
is the simplest G4 chip in this case). It may only be useful to emulate 
more complex G4 CPUs if those have additional instructions which QEMU has 
(or can have) optimised emulation for (and even then only when OS X 
actually uses them which is not likely if it wasn't optimised for these 
CPUs). But I'm not sure this is the case here. Does the later G4 chips 
have any ISA differences that could bring more speed in emulation or are 
they just different in hardware implementation? If it's the latter than I 
think making them work in QEMU would not bring any performance improvement 
and the reason they don't work is likely that QEMU does not (or not 
correctly) emulate some of the added complexity these chips have. But 
someone correct me if I'm wrong, I don't know much about these CPUs but 
I'd like to first see the point of this work. Otherwise there might be 
better places to put effort than fixing emulation of rare CPUs with OSes 
not meant to work with them.

> Balaton, I was trying to boot 10.4.11 with the 7448, so the kernel 
> should support it.

Why do you think so? According to 
https://everymac.com/systems/by_processor/powerpc-g4-powerpc-7400-macs.html 
this CPU was not used in any Mac. Why would've Apple added support for it?

Regards,
BALATON Zoltan