[klelatti]

Short answer is yes. Just one significant example all instructions 32 bit long and no Thumb.

If you read Patterson and Hennessy (Arm edition) there is a slightly wistful throwaway comment I think that Aarch64 has more in common with their vision of MIPS than with the original Arm approach.

Elsewhere you've commented that it's more similar to x86 -> x64 than x86 -> Itanium - which may be true but Itanium was a huge change. However, Aarch64 is philosphically different to 32 bit Arm so it's not really like the x86 -> x64 at all which was basically about extending a 32 bit architecture to be 64 bit.

[mumblemumble]

There's a sort of category problem underlying what you're saying though, perhaps fueled by the fact that ARM has more of a mix-and-match thing going on than Intel chips do.

aarch64 isn't really an equivalent category to x64, because it describes only one portion of the whole ARMv8 spec. ARMv8 still includes the 32-bit instructions and the Thumb. I realize you did mention Thumb, but you incorrectly indicated that it doesn't appear at all in ARMv8. As a counterexample, Apple's first 64-bit chip, the A7, supports all three instruction sets. This was how the iPhone 5S, which had an ARMv8 CPU, was able to natively run software that had been compiled for the ARMv7-based iPhone 5.

A better analogue to aarch64 would be just the long mode portion of x64. The tricky thing is that ARM chips are allowed to drop support for the 32-bit portions of ISA, as Apple did a few years later with A11. Like leeter said in the sibling post, though, x64 chip manufacturers don't necessarily have the option to drop support for legacy mode or real mode.

I think that's a fairly important distinction to make for the purposes of this discussion. I wasn't ever really talking about just aarch64; I was talking about all of ARM.

[klelatti]

> Not only is it an incumbent switching to another architecture; it's an incumbent switching to another incumbent architecture. ARM is older than PowerPC and almost as old as the Macintosh itself; it came out in 1985.

> I gather that it's true that ARM hasn't been as good about backwards compatibility as some of its competitors, but was ARMv8 really so much of a jump from ARMv7 that one can't count it as part of the same line of processors anymore?

> I wasn't ever really talking about just aarch64; I was talking about all of ARM.

M1 is AArch64 only. You incorrectly brought ARMv8 into the discussion. AArch32 is irrelevant in the context of the M1.

Fair to highlight worse backwards compatibility but then you can't bring back AArch32 which Apple dropped years ago to try to claim that the M1 somehow uses an old architecture.

[mumblemumble]

> AArch32 is irrelevant in the context of the M1.

Is it? It's not like Apple moving MacBooks to M1 happened in a vacuum. M1 is only the latest in a whole series of Apple ARM chips, about half of which were non-aarch64.

That context actually seems extremely relevant to me; it demonstrates that Apple is not just jumping wholesale to a brand new architecture. They migrated the way large companies usually do: slowly, incrementally, testing the waters as they go. And aarch64 was absolutely not involved in the formative stages (which are arguably the most important bits) of that process. It hadn't even come into existence yet when Apple released their first product based on Apple Silicon. Heck, you can make a case that the process's roots go way back before Apple Silicon, all the way back to ~1990, when Apple first shipped the Newton.

Note, too, that the person I was originally replying to didn't say "M1", they said "Apple Silicon." In the interest of leaving the goalpost in one place, I followed that precedent.

[klelatti]

Your point now seems to be that M1 is the latest in a line of processors with ISAs designed by Arm limited. I'll agree with that and leave it there.