Home >> DIY Modding Parts >> Sega Master System M404 Patched BIOS

What It Does...

The Master System M404 prototype BIOS was found in a development machine which was originally sent to Sega of America.

Only two games are known to boot with this BIOS: the Japanese version of Fantasy Zone, and Choplifter. These are known to be two of the earliest games developed for the Master System.

It has a very unique starup sequence that was a lot different compared to retail units.

As mentioned above, it can only play 2 known games.....BUT.....After several weeks of work dissasembling the code, we have developed a patched version that boots ALL games, so you can now enjoy this wonderful startup sequence on your own console!

It has been tried and tested on a Master System 2, it is likely it will work on a Master System 1, but we cannot guarantee this.

We are selling a replacement BIOS chip pre-flashed with this new BIOS with fitting instructions below, we are also supplying a 28 pin IC socket with this.

The replacement BIOS is compatible with all Master System 2 consoles with a 28 pin BIOS chip already fitted, it may be compatible with pre-fitted 32 pin chips, but some re-wiring may be needed, the BIOS chip is shown below so you can check yours.

We have spent a lot of time working on this and would hate to see it being copied and sold elsewhere, we have security marked all the chips with a security label and embedded code that is unique to each purchaser, If you have purchased one without the security label, it is a cloned unit, please let us know if this is the case.

Disclaimer:A small handful of games may not work with this BIOS due to incompatibilities, but none found so far.

Disclaimer: We are charging you for the cost of the replacement chip and the time taken to do this work, We are not charging you for the BIOS itself.

Disclaimer: We take NO responsibility for failed installs which renders the console in-operable, we do however offer an installation service.


1. We will be concentrating on removing the following chip shown in RED marked as IC2.

2. Locate and remove the OLD bios chip using a desoldering pump and/or solder wick, ideally it is best to try and preserve the old chip incase you want to change it back at a later date.

3. Install the 28 pin IC socket (provided) in it's place and solder it down and this will make it easier to swap chips back if needed.

4. Install the chip by pressing it down into the socket firmly, making note of the DOT on the chip which should be facing to the back of the console.

5. Turn on the console and enjoy!


Price Each: £15.00

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