[chasil]

Is it not true that the instruction decoder is always active on x86, and is quite complex?

Such a decoder is vastly less sophisticated with AArch64.

That is one obvious architectural drawback for power efficiency: a legacy instruction set with variable word length, two FPUs (x87 and SSE), 16-bit compatibility with segmented memory, and hundreds of otherwise unused opcodes.

How much legacy must Apple implement? Non-kernel AArch32 and Thumb2?

Edit: think about it... R4000 was the first 64-bit MIPS in 1991. AMD64 was introduced in 2000.

AArch64 emerged in 2011, and in taking their time, the designers avoided the mistakes made by others.

[daeken]

There's no AArch32 or Thumb support (A32/T32) on M-series chips. AArch64 (technically A64) is the only supported instruction set. Fun fact: this makes it impossible to run Mario Kart 8 via virtualization on Macs without software translation, since it's A32.

How much that does for efficiency I can't say, but I imagine it helps, especially given just how damn easy it is to decode.

[averne_]

It actually doesn't make much difference: https://chipsandcheese.com/i/138977378/decoder-differences-a...

[chasil]

I had not realized that Apple did not implement any of the 32-bit ARM environment, but that cuts the legs out of this argument in the article:

"In Anandtech’s interview, Jim Keller noted that both x86 and ARM both added features over time as software demands evolved. Both got cleaned up a bit when they went 64-bit, but remain old instruction sets that have seen years of iteration."

I still say that x86 must run two FPUs all the time, and that has to cost some power (AMD must run three - it also has 3dNow).

Intel really couldn't resist adding instructions with each new chip (MMX, PAE for 32-bit, many more on this shorthand list that I don't know), which are now mostly baggage.

[theevilsharpie]

> I still say that x86 must run two FPUs all the time, and that has to cost some power (AMD must run three - it also has 3dNow).

Legacy floating-point and SIMD instructions exposed by the ISA (and extensions to it) don't have any bearing on how the hardware works internally.

Additionally, AMD processors haven't supported 3DNow! in over a decade -- K10 was the last processor family to support it.

[chasil]

80-bit x87 has no bearing on SSE implementation.

Right. Not.

[daeken]

Oh wow, I need to dig way deeper into this but wonderful resource - thanks!