MegaCD2 Japan Bios on PAL MegaCD2

l_oliveira · Post by **l_oliveira** » Sat Jan 10, 2015 2:16 am

TmEE co.(TM) wrote:I'm not seeing how that would work... BIOS in CD side is always visible...

/CART high makes it go to $400000... No conflicts.

Tomorrow I'll try to implement it with just a 74LS138... If it works like I think it does, there won't even be issues with mirroring or anything similar.

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sat Jan 10, 2015 3:04 am

Now I see how it would work, I had a brainfart.

You will have to deal with TMSS first though, otherwise it gets in the way.

l_oliveira · Post by **l_oliveira** » Sat Jan 10, 2015 3:08 am

TmEE co.(TM) wrote:Now I see how it would work, I had a brainfart.

You will have to deal with TMSS first though, otherwise it gets in the way.

Not really, my Mega Drive is a VA3. And I'm going to only test patches and the translation of the Japanese BIOS to English I wanted to do since forever ... lol

l_oliveira · Post by **l_oliveira** » Sun Jan 11, 2015 6:34 pm

(wip)

I am still working on the logic which toggles /CART HIGH when the HINT vector is read so the MCE chip has control of HINT address. (that's what the 7430s being drawn will do when this is completed)

It already boots and works (mostly) fine with just the 74LS138 G1 pin connected to VCC.

I booted the USA 1.10 SEGA CD ROM this way on a TMSS VA6 MD1.

The unmodified SEGA CD BIOS satisfied the TMSS check and stuff worked correctly.

The wires going to the 68000 are there because the pirate board I am using for this don't have the pads for those address lines. :\

TmEE co.(TM) · Post by **TmEE co.(TM)** » Sun Jan 11, 2015 8:32 pm

Neat

l_oliveira · Post by **l_oliveira** » Mon Jan 12, 2015 3:03 am

This is the simplest I could get with the parts I have here:

Tomorrow I'll assemble that and debug/change if there's any mistakes with address decoding.

For now I found out a few games which fail without the HINT remapping from the MCE chip:

Snatcher (menus fail to load the fonts)
Silpheed (crashes on the first frame of video playback)
Sengoku (crashes before entering gameplay)

Hopefully these work ok after I complete the design.

notaz · Post by **notaz** » Mon Jan 12, 2015 1:50 pm

l_oliveira wrote: I booted the USA 1.10 SEGA CD ROM this way on a TMSS VA6 MD1.

The unmodified SEGA CD BIOS satisfied the TMSS check and stuff worked correctly.

Nice, wasn't expecting it to handle that as it doesn't have to.

l_oliveira · Post by **l_oliveira** » Mon Jan 12, 2015 1:54 pm

Apparently KRIKzz is unaware of the HINT stuff (register $A12006 and vector $000070) thus Mega Drive everdrives glitch a lot on CD stuff:

http://krikzz.com/forum/index.php?topic=1936.0

Chilly Willy · Post by **Chilly Willy** » Mon Jan 12, 2015 9:08 pm

Nice work! I popped off a PM to KRIKzz to check out the thread. Hopefully this will help him with better CD support on his carts.