This toot https://fosstodon.org/@interlisp/115122557055111398 led me via a reference to Edmund C. Berkeley (as coauthor with L. Peter Deutsch of "The LISP Implementation for the PDP-1 Computer", March 1964 and author of "Giant Brains, or Machines that Think", 1949) to the 1950 2-bit relay computer Simon and then to Harry Porter's Relay Computer:

https://www.youtube.com/watch?v=tsp2JntuZ3c

By eighteen minutes in, if you can follow along, you'll understand how ALU function codes are selected.

I've never seen such a succinct explanation of a computer.

[Edit: the 3-bit ALU function codes are later wrapped in 8-bit opcodes, which trigger loads and selects from the data and address buses. This is exposition at its finest!]

#computerarchitecture #relaylogic #vintagecomputing #retrocomputing