In the early days the CPUs had loadable microcode. With new operating system versions came language extensions for Lisp. Then one also got a new version of the microcode, which then had been extended for, say, inclusion of an instruction for OOP method dispatch. One also could compile some code to microcode.

Later the CPUs were actually Microprocessors (from TI and Symbolics). At least for the Symbolics Ivory microprocessor, the microcode no longer was loadable.