Now I'm curious whether Joe Armstrong's original Prolog implementation of the VM is available anywhere, but I doubt it.
See also The Erlang BEAM Virtual Machine Specification by the OG Bogumil Hausman himself ! Note: This document describes BEAM as it was in 1997. BEAM has grown and changed significantly between then and the time this note was added (2012). This information is mainly for historical interest. - https://www.cs-lab.org/historical_beam_instruction_set.html
Together, they should provide a lot of insights into ERTS/BEAM.
I hope that's preserved and one day published as e.g. the old MIT AI lab file system snapshots were.
(Robert Virding or Bjärne Däcker might well have a copy of the Prolog code to share if asked nicely.)