MegaCD2 Japan Bios on PAL MegaCD2

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MegaCD2 Japan Bios on PAL MegaCD2

Post by Orion_ » Sun Oct 12, 2014 3:09 pm

I have an Everdrive, and a Megadrive 2 + MegaCD 2, both PAL.
I found that I can boot the Japanese MegaCD 2 Bios using the everdrive, so I bought some Japanese games and some are running very well (time gal, ax-101)
Unfortunately, some other games aren't :(
For example, microcosm and starwars won't boot -> black screen, or starblade will start, show title screen, then hang when starting a new game.
I tried to mod my megadrive with a 50/60hz switch, but even in 60hz mode, it won't fix the problem.
Do I need to add the usa/japan switch ? or any other modification ?
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Post by Orion_ » Wed Oct 15, 2014 10:21 am

Did I post in the wrong section ? Or nobody ever tried to boot another MegaCD Bios on everdrive ??
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Post by Eke » Wed Oct 15, 2014 3:14 pm

Some games will check the console region on startup so you will need a region switch on your Mega Drive (I don't think the Everdrive can patch it on the fly).

Most games do not check the 50/60hz setting so a 50/60hz switch will only affect VDP timings.

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Post by oofwill » Sat Oct 18, 2014 11:20 am

Yes! that's it.

Jap bios accept EUR megadrive system. It's surprising.
But certains games are checking MCD region AND MD region. You'll have to modify your MD to play those games...

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Post by Orion_ » Thu Jan 01, 2015 4:08 pm

I switched my Megadrive with both 50/60Hz + USA/Jap switch.
I start the Megacd 2 Japanese Bios from the Everdrive in 60Hz/JAP mode.
And the 3 games that weren't working before (in 50Hz/PAL) still don't work in 60Hz/JAP :/
So, I guess, this might be an everdrive incompatibility.
Jap Games not working: Starwars rebel assault, Microcosm, Starbalde
Jap Games working (in any mode): Time Gal, AX-101
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Post by Stef » Fri Jan 02, 2015 12:30 pm

I have to admit i never tested these games but i don't see how can it be an everdrive incompatibility. The Everdrive basically maps the loaded BIOS to the usual MegaCD BIOS area so at this point it should be 100% compatible.
If the issue is region, you should have a message anyway...
I would say that at soon you Megadrive is correctly clocked (60 Hz) then it should work. The MegaCD clock stay the same whatever is the region.

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Post by Eke » Fri Jan 02, 2015 3:36 pm

It could be an incompatibility between the "BIOS" you used and your CD hardware, try with another revision or with a "region-free BIOS" for your system.

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Post by Orion_ » Fri Jan 02, 2015 4:30 pm

I have a PAL Mega CD 2, so I used Japan Mega CD 2 bios.
I tried a the "EU Mega CD 2" region free bios, it doesn't work with any Jap game I have (reboot after loading the boot)
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Post by Mask of Destiny » Fri Jan 02, 2015 7:27 pm

How does the Everdrive work for this case exactly? I'm not aware of any mechanism that would allow the rest of the CD hardware to be mapped appropriately, but leave the BIOS area unmapped and it's not like it sits between the original BIOS ROM and the 68K so it can't intercept the read. Is it just driving the bus harder than the ROM on the board?

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Post by TmEE co.(TM) » Fri Jan 02, 2015 10:02 pm

I don't see any other mechanism possible besides making a bus fight and winning it.
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Post by raq » Sat Jan 03, 2015 2:07 pm

Isn't it the case when a cart is inserted the CD BIOS (the original one in the Mega-CD) gets mapped to 0x40000000 and cart is mapped to 0x00000000 and this swaps around when cart is removed?

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Post by notaz » Mon Jan 05, 2015 1:54 pm

Mask of Destiny wrote:How does the Everdrive work for this case exactly?
Maybe by watching address bus and toggling CART_IN during BIOS access?
raq wrote:Isn't it the case when a cart is inserted the CD BIOS (the original one in the Mega-CD) gets mapped to 0x40000000 and cart is mapped to 0x00000000 and this swaps around when cart is removed?
toooo many zeroesss 0x40000000
And with that you get CD's RAMs and stuff mapped at 400000+ too, which is not what the BIOS expects.

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Post by raq » Fri Jan 09, 2015 7:45 pm

ooops, my mistake on too many zeroes :oops:

There is a bit in the config register of the everdrive thats set when a BIOS is flashed to the everdrive, so maybe the fpga traps addresses 0x20000 - 0x3ffff and 0x200000 - 0x23ffff and sets CART_IN high when in this range.

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Post by l_oliveira » Sat Jan 10, 2015 1:35 am

raq wrote: There is a bit in the config register of the everdrive thats set when a BIOS is flashed to the everdrive, so maybe the fpga traps addresses 0x20000 - 0x3ffff and 0x200000 - 0x23ffff and sets CART_IN high when in this range.
That gave me a idea for testing modded CD BIOSES without opening the Mega CD ! :D

I was curious about how this worked and I even asked Tiido about this. He said he thought it fought the BUS using the 68000 bus control lines. But using /CART makes a lot more sense in the end.

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Post by TmEE co.(TM) » Sat Jan 10, 2015 1:48 am

I'm not seeing how that would work... BIOS in CD side is always visible...
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