I should probably have written what I thought:

Tobias Geerinckx-Rice wrote on 08/01/18 at 22:51:
> AIUI, at least on x86 CPUs, the microcode *is* a large and/or functional
> part of the processor...

...but it's initially included in ROM. Only when bugs are found in that
copy does a user-provided ‘update’ (at every boot) become needed.

Kind regards,

T G-R