Region Free BIOS (Sega Mega CD)

if you have a single specific bios, such as an eu region free then you can just install it in place of the original multibios if you have the region free multibios then it can be installed in place of the original bios and have its pins 38 (a16) and pin 39 (a17) toggled to control which bios to load if they are sent to ground or 5v our multibios has the following bank order mega cd 1 bank 0 mega cd 1 00s (jp) pal dec 28 1991 38 gnd / 39 gnd bank 1 mega cd 1 00s (jp) dec 28 1991 38 5v / 39 gnd bank 2 mega cd 1 00 (e) oct 27 1992 38 gnd / 39 5v bank 3 sega cd 1 10 (us) oct 11 1992 38 5v / 39 5v mega cd 2 bank 0 mega cd 2 00c (jp) dec 22 1992 38 gnd / 39 gnd bank 1 mega cd 2 00c (jp) dec 22 1992 38 5v / 39 gnd bank 2 mega cd 2 00w (e) b1 jun 01 1993 38 gnd / 39 5v bank 3 sega cd 2 00w (us) jun 01 1993 38 5v / 39 5v this means you can wire pin 38 to video rate pin (ground 50hz, 5v 60hz) pin 39 to language (ground japanese, 5v english) mapping the consoles actual region settings to the right bios console settings the mega drive / genesis has the following video rate and language jumper voltages that control the systems mode region video rate language invalid (technically japan pal) ground (50hz) ground (japanese) 60hz/jp (japan) 5v (60hz) ground (japanese) 50hz/en (europe) ground (50hz) 5v (english) 60hz/en (north america) 5v (60hz) 5v (english) pass through switching mod if you are using a switchless region mod, it is common to make use of the unused pins on the interconnect edge connector on the mega drive and mega cd to do this use b25 and b27 unused pins, solder wires from your switchless mod on the mega drive to those pins, open up the side connector pcb and jumper top to bottom the b25 and b27 pins too so they have connection between the mega drive and mega cd now, then solder a wire on the mega cd pins b25 and b27 to the bios bank pins 38 and 39