Fortunately, the Watcom compiler (tool of choice at the time) had a mechanism for describing register use and side effects of embedded code fragments - somewhat

Fortunately, the Watcom compiler (tool of choice at the time) had a mechanism for describing register use and side effects of embedded code fragments - somewhat like gcc. It was common for any 3d games to have 16.16 fixed point type all wrapped up in macros using asm fragments. The crucial thing was using a 64 bit intermediate for muldiv and mulacc expressions.