Have you ever seen a USB stack (even for HID boot protocol) in assembly on a commercial BIOS?
Yes. On AMIBIOS 627.10 and Award v6.00 (both used into the early 21st century) the USB stack is 100% Asm, as is the rest of the BIOS.
I left that scene in the late 2000s but my understanding is that C didn't start showing up in BIOS until the UEFI era (Tianocore etc.).
There have been some leaks over the years of both "old-school" and "new-school" BIOSes, so these facts can be verified.
I will take a look, but right now I would be skeptical of any such "100% asm" "leak", considering how easy it is to get a lightly commented listing. My position is indefensible, so no point arguing though.
> C didn't start showing up in BIOS until the UEFI era (Tianocore etc.).
"Showing up" is an understatement, since Tianocore is almost all C. And we didn't switch from 100% asm to 99% c overnight.