Oops, I should have thought twice before asking this question... I found the answer myself As a repent , for those interested here is what I added: in driver/neogeo.c, under SYSTEM_BIOS_START( neogeo ) SYSTEM_BIOS_ADD( 7, "uni-bios.12","Unibios MVS (Hack, Ver. 1.2)" ) and under #define NEOGEO_BIOS \ ROM_LOAD16_WORD_SWAP_BIOS( 7, "uni-bios.12", 0x00000, 0x020000, CRC(4fa698e9) ) /* Universe Bios v1.2 (hack) */ and in ui/properties.c in InitializeBIOSUI(HWND hwnd) (near the end, after the other ComboBox_AddString) ComboBox_AddString(hCtrl,"Unibios MVS (Hack, Ver. 1.2)"); Next time I'll try harder to find the answer before asking the question