Sega CD multibios - anyone ever made their own?

[FABombjoy]

Well, I've tried but it didn't quite work, and I'm just not sure why.

I downloaded the 3 BIOS images for the 3 regions, catted them together (doubling the end image), so it was something like:

US
JP
EU
EU

...and burned the image to a 27c4096.

I piggybacked the new chip on the original bios, and pulled the originals CE high.

On the new chip, I lifted and tied pins 38 and 39 to ground (via 15k resistors). CE tied to the board at original location.

Result: Nothing at all. I jumpered 38 and 39 to +5 and still no reaction. I can rearrange things and get the stock BIOS to work just fine, but I can't get this chip to come alive. I went back, restored the CE lines, and tied the stock BIOS's OE high (adjusting the new BIOS accordingly) and still nothing.

My understanding is that it should either worked OR brought up a screen telling me that I'm using a Mega CD with a Genesis (or something like that).

Perhaps it just doesn't like the particular EPROM that I'm using? It's this part: http://www.atmel.com/dyn/resources/prod_documents/doc0311.pdf.

Any theories, ideas, or criticisms?

[chadtower]

I'm not sure I quite get what you're doing. I see you've put all three bios onto one chip. Are you going to be manually switching between them? I assume you can't just slap all three end to end and assume they're going to autodetect. Maybe you could write a tiny front end to them, though, that would do autodetection of the region of the game and then boot to whatever is the appropriate bios image.

[FABombjoy]

Yes, there will be a switch (once I have something functional in place). A17 and A16 are pulled LOW to select a single 128K bank

[chadtower]

Nice. I don't have much more to contribute, but nice. LOL

[FABombjoy]

Well, it turns out that the available BIOS images have been hi/lo byteswapped. A spin through a re-swapper program, a fresh new PROM later... and I now have a 3-region Sega CD unit.

[googlefest1]

nice - is there any problems displaying PAL/NTSC

[FABombjoy]

Yes and no... When the Genesis is jumpered for US/50Hz, the Mega CD screen will roll and the color disappears. If you boot it that way and change it back to 60Hz while it's still running, the pictures stabilizes and the color returns. However, there is some unusal screen flicker. I don't have any PAL games to test it with, so I can only go by the Mega CD intro screen.

[murdiaries]

You're great man!! :D

I am trying to do the same on my Japanese Mega-CD, but need help in order to successfully do it..

The 27c4096 worked for you in the end?
Which programs and how do you use them for reswapping downloaded MCD's bios? and for flashing eprom?

If you could assist me in doing the multibios mod it would be so much appreciated!! Thanks!!

[FABombjoy]

Yep, I've done several now and use the 27c4096.

I use a single switch to select the region, although some people prefer to use 2. BYTESWAP.EXE will convert the downloadable images for burning. If you open a BIOS image in a text editor and can read text like "SEGA MEGA CD" (or something like that), then it needs to be byteswapped so it reads something like "ESAGM GE ADC".

For eprom flashing, just use whatever your eprom reader came with. Depending on what burner you have, you may need a 16-bit / 40-pin eprom adapter.

[murdiaries]

Thanks!!
I'm also trying to have a custom screen on Bios startup, so far there are only small palette changes and little tile work.

One question comes in my mind, is it also possible to remove the region check from bios? If so, we could have just one custom universal bios.
http://img112.imageshack.us/img112/6632/58045128mm5.gif

I'll post my experiments here, thanks! ;)