[rrss]

WMMX was exposed via the ARM coprocessor mechanism (so it was permitted by the architecture). The coprocessor stuff was removed in ARMv8.

[my123]

Now custom instructions are directly on the regular instruction space...

(+ there's the can of worms of target-specific MSRs being writable from user-space, Apple does this as part of APRR to flip the JIT region from RW- to R-X and vice-versa without going through a trip to the kernel. That also has the advantage that the state is modifiable per-thread)

[Followerer]

In ARMv8 you have a much cleaner mechanism through system registers(MSR/MRS).

[saagarjha]

Apple has been using system registers for years already. AMX is interesting because it's actual instruction encodings that are unused by the spec.