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Comment by iso8859-1 | Hacker News Reader

Since all modern CPUs have a ton of microcode, would it be hard to emulate the x86 when needed, and retain backwards compatibility like that?

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Not particlarly; in fact, apple has basically done this exact thing twice in the past (https://en.wikipedia.org/wiki/Mac_68k_emulator), (https://en.wikipedia.org/wiki/Rosetta_(software).

But I believe we're (hypothetically) talking about ARM, not x86, code.

Aside: Apple's 68k-on-PowerPC emulator was an amazing piece of work. It didn't run 68k code in a separate environment; it allowed 68k code to run alongside PowerPC code, to nearly the same level of transparency. Applications could mix ISAs in libraries and code resources, and system extensions could patch PowerPC system functions using 68k code or vice versa. It was surprisingly efficient, too: the first PowerPC systems used system software consisting mostly of 68k code, and still ran faster than the native 68k systems they replaced.

I've never seen anything quite like it since. About the closest I've seen is the way that Thumb code can be integrated with native ARM code, and that's explicitly just a different instruction encoding, rather than a separate ISA.

A big part of the success with the 68k switch over was that the 68k line was getting long at the tooth. Clock for clock 601 would trounce 68040 (even the 486 was faster per clock). Plus the base 601 was at 60Mhz, while the fastest 040 based mac was 40Mhz (the quadra 840av, of which I picked up one a few years later for $40, and still have it).

So apple was working with something north of 3x the raw performance, its no wonder that most people with older macs that weren't anywhere near the performance of the 840AV thought the powermac was incredible.

The problem with that, at least in the case of Rosetta, is that the QuickTransit technology underlying it got bought by IBM and disappeared from the open market, so doing that trick again would be...a moderate time+cost sink.

Wiki for QuickTransit seems to think that a number of prominent people from Transitive hopped to ARM and Apple (which might be telling, given the claims of the original post), but has no citations.

Eh, JITs aren't the most complicated things on the planet. There's plenty of people who can do it outside of QuickTransit. Like, look inside most emulators.

Given that Apple is writing compilers and designing chips, on-the-fly recompilation shouldn't be a big problem for them.

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