Region free BIOS for MEGA-CD Version 2.00

Thanks for this awesome work!
Do i still have to byte swap before burning?

Normaly i would use a 27c1024. But can i also use a 27c210 like in the NeoGeo with UniBIOS?

 

Exactly same thing as with the UniBIOS. PC is little endian so the byte ordering for storage is byte reversed from what the 68000 expects.
The files in my archive are in little endian format because "mega drive roms" are usually shared on that format.

So, yes, you need to byteswap. 27C1024 and 27C210 are technically the same thing after they have been programmed.

 

Thanks again!

 

What's the "design issue" you refer to about the EverDrives?

Just curious.

 

It overrides the MEGA-CD ASIC by manipulating the cartridge detection pin. It would not be a issue if the said ASIC did not have a special function which allows it to redirect the VDP(video chip) horizontal interrupt handler from the Mega Drive CPU to a spot on the work ram.

See this thread where headcrab was being baffled by it: http://www.assemblergames.com/forum...cts-MegaCD-kit&p=776526&viewfull=1#post776526

All the games which fail the way they do, do fail because they actually use the HINT vector redirection function. The BIOS defaults that interrupt vector to to "FFFD0C", but a game can set it to be anything within the address space allotted for the Mega Drive console work RAM.

That doesn't work on Everdrive because it redirects the BIOS memory range completely to the cartridge slot, defeating the masked out word and blocking the reprogramming of the HINT vector. So in the end the HINT will be serviced at FFFD0C but only God knows what will be at that address (since the games which do fail likely put their HINT handling code elsewhere and count with the remapping working).

 

Thanks for the explanation, this is great.
I did find a couple of games that didn't run on the CD region free BIOS but worked fine on the stock one, at the beginning I thought it was a CD BIOS free issue and not the way ED hijacks HINT.

Now so told, would there be any other way to achieve the same result without hijacking HINT and still have region free bios loaded in the Everdrive? From what you pointed me at it doesn't look like.

 

Fit the region free BIOS inside the CD unit. For now that's the only way. This information has been forwarded to Krikzz so it's up to him how (or if) he will deal with it.

So far the only *valid* claim of a game not working with the region free BIOS was "Flashback: The quest for identity" which doesn't run on the Japanese BIOS (apparently it uses the same trick the Japanese games use to not run on the non Japanese BIOSes).

 

I am trying to byteswap the bios so I can flash to an eeprom and would just like to check I've done everything right before flashing...

I used Hex Workshop to open the .bin file then, under 'byte order', selected 'Big Endian (Motorola)' and saved over the original file.

Will this be okay? There is a 'byte-flip' option but it appears to be greyed out.

 

if you open the bin file and you can't read the text:

"SEGA" stored in big endian way would become "ESAG" when read on a little endian system.

So you will see:
ESAGM GE ARDVI E
C(S)GE A9139S.PE
DC XOBTOR MO
0 /9701-99 3 910: 0 2 2.X1
DC XOBTOR MO
RB0 0000-6.212

for:

SEGA MEGA DRIVE
(C)SEGA 1993.SEP
CDX BOOT ROM
09/07-1993 19:00
2.21XCDX BOOT ROM
BR 000006-2.21

after byteswapping.

 

Way Past Cool!
Thank you- the Sega CDX US hacked ROM worked like a charm for playing my PAL Sonic CD A tiny, tiny bit of lag but I think that's just cause it's meant to be 50hz.

Anyone wanna sell me a programmed SOP EEPROM with one of these so I can install it my modded CDX? it would be so sweeeeet.

 

What's the best region free bios to use?
Should we just use the JP BIOS and be done with it or is there a list of games that no matter region free or not they only work on the right BIOS?
In specific between a US or JP BIOS for US MegaCDs is there any reason to prefer one over the other? (outside personal liking of the opening music and such).

 

The Japanese one has the best compatibility rate.

But in the truth some games even go down to check the version and region of the BIOS. For example USA/PAL FLASHBACK will not work on the Japanese BIOS. It checks the region of the SUB CPU BIOS. -__-;

 

Assume I have a US Sega CD1, is there any reason NOT to replace the BIOS with a dual BIOS chip "region free US + region free JP"?
That is to ask is there any game that works on stock BIOS that has issue with your region free version of it?
If the US region free BIOS has no issue with any US games then I see no reason to keep a stock US BIOS at all.

 

That would be the best solution I suppose. Like I mentioned on the post you quoted, some games actually check what BIOS the machine has so, there's no real solution.

So there's really no 100% solution besides maybe hack the games to remove the protections.

 

I wonder why a developer would bother to make checks so deeply just to avoid playing in another region.

 

^^^^^^ BINGO!! I just got a version 1.00 Mega CD Bios installed in my model one, easiest install since it was a first run with a socketed bios, i haven't noticed anything crazy yet, that flashback issue has me a little puzzled, might have to download that one and see if you can remove the protections.

You would think with how shitty the Sega CD/ Mega CD sold they wouldn't go through such great lengths to sabotage something that could add longevity to a console......

 

x

 

That question has been answered a million times over and it has all to do with making money.

 

If I did that, a Japanese BIOS would effectively become a European/USA BIOS. And then it would stop working with the Japanese games which have locks in them.

What is checked really depends on the game in question. For example Wolfteam games check RAM for leftovers of the code which shows that animated SONIC logo on the screen. Japanese BIOS show just a simple, small monochromatic SEGA logo in blue. So that skip SONIC logo feature I added make Wolfteam games work. That's why I put it into the PAL/NTSC BIOSes.

A different kind of protection is this "Ishii Hisaichi No Daisekai" game which explicitly checks the SUB CPU BIOS region flag. (that JUE string on the header)
I did not edit that yet but I suppose that would not hurt compatibility wise... Maybe save that for the next release if there's any.