Interesting...
I guess I'm basically clueless in this area, but I assumed that the hardware designers, especially back then, were focused on minimal hardware/ROM/etc. resources, and had to make their code as efficient and minimal and directed towards the specific manufacturers' A/D converters and other hardware they were using in the particular piece of equipment.
Which implies to me that they would have some absolutely bare-bones and custom, vendor-specific assembly-type code, or at most maybe a tiny subset of a full-on language like a C interpreter or whatever.