It sounds like this machine had a CPU with two instruction sets: one general purpose and the other for display list processing.
From retrocomputing stack exchange:
The Imlac PDS-1 is exactly what you’re looking for: A general purpose computer with two instruction sets - one instruction set was a “stretched” PDP-8 (16 bits instead of 12 bits, 2 registers instead of 1) and the other complete instruction set - with subroutine calls! - was for executing display lists on the built-in vector graphics console.
More Digibarn videos on this machine here: