Mega CD/Sega CD Switchless Multibios

I really liked my Mega Drive 1 with Sebs Switchless Saturn mod fitted, I also really like Aarakons MultiBIOS idea.  So I wanted to combine the two.  I started by downloading some un-modified Model 1 MegaCD BIOS files, and with some advice from Aarakon I compiled my own MultiBIOS. Then studying Sebs Saturn Mod I got them talking to each other!

This page assumes that you have already modded your MD with the Switchless mod, and that you have fitted the socket to your MCD1 as per this, finally - you have burned the downloaded code to the EPROM.  This page may not be as detailed as some of my others, sorry - I did the mod about a year ago but never got a chance to publish it.

Notes and photos:

Switching to JPN mode freezes the console up, I have to power off and back on to use JPN mode (maybe there a later revision of JPN bios that might fix this?), I remember Andy from ConsolePassion who did his own version of this mod saying his does the same also.  UPDATE - I tried all the Japanese BIOS files I could find, it froze with all of them.  Then I played with an unmodded Japanese MegaCD, reset doesn't eject the tray, you have to eject using the on-screen menu.

This mod always ejects when switching region, I guess because swapping regions on the switchless mod resets the console, but with a MegaCD 1 hooked up the reset button usually ejects the tray so this isn't much of a problem.

Order of the bios files in my MegaCD MultiBIOS switchless.bin

USA --> JPN --> EUR

Why that order?  Well this guide uses legs 8 and 10 from Sebs Switchless Saturn Mod, combining the BIOS files in that order works really well with Sebs code and needs none of the Switchless mod code altered:

  MegaCD Mega Drive
      Language Video Mode

16F630 output leg

8 10 9 12

Connects to

38 on Bios 39 on Bios JP 1/2 JP 3/4

Output from Sebs Switchless Mod


GND +5v +5v GND


+5v GND GND +5v


GND GND +5v +5v

How to get the signal from the 16F630 chip from the MD to the MCD?  There are some unused contacts on the exp bus connector (near the right hand side)

Here you can see the red/black and white/blue wires coming from the switchless mod fitted in my MD

Red and White wires, legs 8 and 10

The wires are connected to the 2 unused contacts on the Expansion bus, top-side of the motherboard, to the right of the connector (try to solder as far away from the edge as possible so that it doesn't interfere when plugging it all up together)

 Upper side of MD board

The unused connectors are not linked up in the MD - MCD connector, so take that apart and solder two wires in (if I had followed my colour coding the wire on the left would be the red/black one, the wire on the right would be the white/blue one)

Linking up through the connecting block

Finally, you can see that I've linked up the equivalent unused contacts on the MegaCD1 motherboard with the lifted legs of the MultiBIOS socket (red/black to leg 38, white/blue to leg 39)

 Legs 38 and 39 wired up

Summary of Components needed

1 x Switchless MultiBios (this is on either a 27C4096 or 27C4002 EPROM)
1 x 40 pin DIP socket fitted to your MegaCD as per this guide
some wire
MD with Switchless Mod

I used a Willem Parallel Port Programmer, with a 16-bit adapter and a 12 volt PSU to put the code on the 27C4096 chip.

 !! H E L P !!

Game console modifications