The Power Mac G4 Cube comes with Mac OS 9 installed. For the latest information about Mac OS 9, see the references listed in Mac OS 9.
Like the system software on other Macintosh models since the introduction of the iMac, system software for the Power Mac G4 Cube has Open Firmware booting and Mac OS ROM in RAM. For more information, see the references in ROM-in-RAM Architecture and Open Firmware.