[nudq]

I was under the impression that the pioneering Xerox systems were written in Mesa, which was not even object oriented. I'd love to hear the real story as well.

[pjmlp]

Xerox workstations used microded CPUs, there were Interlisp-D, Smalltalk and Mesa (later Mesa/Cedar) variants of it.

Each variation loaded the respective microcode into the CPU and booted into the respective environment.

You can read all about those environments here https://archive.org/details/bitsavers_xerox