Reverse engineering circuitry in a Spacelab computer from 1980

Spacelab was a reusable laboratory that could be carried in the cargo bay of the Space Shuttle, providing lab space for astronauts and experiments. Spacelab was controlled by a French-built minicomputer, called the Mitra 125 MS. Unlike modern computers, this computer didn't contain a microprocessor chip. Instead, its 16-bit processor was constructed from several boards of chips. In this article, I reverse-engineer one of the processor boards, shown below, part of the computer's Arithmetic/Logic ...