The SC131, pocket-sized Z180 computer has been designed primarily to run RomWBW (which includes CP/M):
The recommended version of RomWBW for this board is RomWBW sc131.
Alternatively, the motherboard can run the Small Computer Monitor.
The recommended version of the Small Computer Monitor for this board is SCM v1.0 configuration S6. This configuration of SCM has been written for SC126 but works just as well on SC131.