I had a 486 with AMI WinBIOS [1]. It looks like a UEFI setup, just 20 years earlier. If they wrote all that in ASM, then I'm really impressed!

https://www.youtube.com/watch?v=2aHWtVJSctM

Yes, WinBIOS was 100% Asm.

It wouldn't fit in a 64K ROM otherwise, and even then it was compressed.