Real Kato Blog: Apple Watch: Is PearPC Real, or a Novelty?

Some enterprising folks have created a PPC emulator, called PearPC, ostensibly to run Mac OS X on x86 platforms. It's an amusing experiment, but it'll never work.

Right now, the creators of PearPC have a disclaimer that their emulator is only in its preliminary stages, and that the emulated processor currently runs 500 times slower than (or 0.2% the speed of) the x86 host. So a 3 GHz Pentium 4 would run like a 4 MHz PowerPC (taking into account processor pipeline depth and whatnot). I think their estimates are generous, so you're really talking about the speed of an Apple II here (you know, the computer that was discontinued twenty years ago).

I looked at their freely downloadable source. It is decidedly an elementary effort, as far as emulation projects go. PowerPC instruction execution is all done with a lookup table that maps a PPC instruction to a C function. So a one-cycle PowerPC instruction requires, oh, a few hundred bytes of program and data store lookup, as well as a hundred or more Pentium instructions. It's easy to see why this is so slow.

Here's the thing about emulating PowerPC on x86 (and why even Apple is unlikely to ever try this): even if you had hand-coded assembly and a dynamic recompiling emulator, the speed of emulation will be terribly slow. You'd probably get about 5% the speed of the host processor (i.e. maybe about the speed of the Macintosh SE), because x86 processors do not have enough registers to emulate the PowerPC registers. A lightning-fast register-based operation on the PowerPC would require main-memory access on the x86. There's just no comparison, speed-wise.

And there's the usual problem of emulating other hardware components, such as video, sound, disk, USB, etc.

By comparison, emulating x86 on PowerPC is simpler, since the PowerPC has registers to spare. In a lot of cases, x86 instructions can be mapped to a single PowerPC instruction. This is why Virtual PC for the Macintosh is still able to achieve about 25% the speed of the host, while fully emulating all I/O hardware as well as the CPU.

Macworld UK has a horrendously ignorant article which suggests that PearPC is a step towards giving PC users a way to run OS X. Hey guys: why don't you actually try it and see how "usable" it is? One screenshot of the Mac OS X boot screen isn't a road map to a working product.

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