The Nintendo 64 ROM is only 2KB in size and apparently easy to emulate. It seems to only check the validity of the inserted cartridge's ROM and set up a limited environment.

Nintendo 64 cartridges are self-sufficient; they don't need any services provided by a common “BIOS”. In fact they even contain the code used to drive the audio and graphics co-processors! Because the games are provided as ROMs on the cartridges, the console needs very little initialisation code — in the same way as its cartridge-based predecessors, and unlike CD or DVD based systems which need a system ROM to load the games off the medium (with a filesystem, encryption etc.).

CEN64's use of the system ROM is in keeping with its accuracy: it aims to emulate all aspects of the hardware, down to the use of the original ROM instead of a high-level emulation.