Microcode

Microcode (Microcode): Data used as part of instruction decode on a microcoded microprocessor to generate one or more micro-operations from each decoded opcode.  

POWER9 CPUs do not have any microcode; all architectural opcodes are hardcoded. There are some auxillary microprocessors on-die to handle power management functions (the CME, SGPE and PGPE, as well as the OCC); the code which runs on these is open-source.

References

• Source code for POWER9 power management cores