Unlike modern consoles that utilize a single unified firmware, the Neo Geo hardware architecture relied on a dual-BIOS system, split between two distinct processors: the Motorola 68000 (CPU) and the Zilog Z80 (Sound CPU).
The BIOS provides default VBlank (vertical blank) handlers for controller reading, coin counting, and watchdog refresh. Many games overwrite these, but the BIOS’s fallback prevents crashes. neo geo bios
On early MVS boards, a BIOS error (or failing cartridge slot) would cause the 68000 to read garbage and execute an invalid opcode. The BIOS’s exception handler, instead of halting, loops an infinite read of the Z80 sound RAM—which the audio CPU interprets as a repeated short click. Hence the infamous rapid clicking sound of a dead Neo Geo. Unlike modern consoles that utilize a single unified