Modding a GBA to behave like a real GBC

Hi,

there's something that's bugging me lately.

As many of you surely know, there are some GBC games that are "GBA enhanced" - when you put them to a GBA, they will make access to an additional content and swap to a brighter color palette so the image is nicely visible on the original GBA.

To my knowledge there are four of those enhanced games:

- The Legend of Zelda: Oracle of Ages
- The Legend of Zelda: Oracle of Seasons
- Wendy: Every Witch Way
- Shantae

Anyway, additional content is nice. But the birghter color palette is not, especially when you're playing on a GBA SP AGS-101 (the one with the better screen) or Gameboy Player (on Gamecube). The colors look washed out in comparion to the original colors.

What I want to do - I want to play those games in their original colors on a GBA SP AGS-101/GBP without using a cheat device such as Gameshark. I have one and I tried to play the Zelda: Oracle of Seasons with it and it works (the cheat codes already exist, see the discussion on Gamehacking.org). There is one problem: the contact with the cartridge is loose and sometimes the game freezes when I unknowingly touch the device while playing. So it's not very practical and certainly not ergonomical as the device is rather large and uncomfortable too.

This is how the detection works (from GbdevWiki):

"Detecting CGB (and GBA) functions CGB hardware can be detected by examing the CPU accumulator (A-register) directly after startup. A value of 11h indicates CGB (or GBA) hardware, if so, CGB functions can be used (if unlocked, see above). When A=11h, you may also examine Bit 0 of the CPUs B-Register to separate between CGB (bit cleared) and GBA (bit set), by that detection it is possible to use 'repaired' color palette data matching for GBA displays."

I made some research and it appears that whether the B register is filled with 0 or 1 depends on the BIOS. I thought that I could swap the BIOS module with a one from a broken Gameboy Color, but unfortunately the BIOS is embedded in the CPU chip. You even cannot update the BIOS as it's read-only.

My question is - is it possible to make some kind of modchip that would force a game to think it's inserted in a GBC when it's actually inserted in a GBA? What would be needed to make it work?

 

Might be easier to get Everdrive and hacked ROM to stay in GBC mode.

 

The thing is I would like to use a real cartridge with a real system.

I'm sure that it's possible. I just don't know how. Yet.

 

It might just be easier to get a backlit GBC. I've seen the mod kits on eBay before and some people are selling backlit systems on there too.

 

That would be the ideal solution. But unfortunately the backlit GBC doesn't exist and won't exist till someone will start making third party backlit screens. There are mods but the screen is frontlit. It's just not the same.

And I must not forget that there is rather nice backlit GBC clone (I can't remember the name - maybe GB Color?) but the screen has not the same aspect ration (the picture looks a little stretched) and what's more important - it just isn't the real hardware.

 

Never go with clones. I heard of people taking the backlit screens from broken DS lites and using those on a Game Boy Color. It might be worth a shot. I can't seem to find a tutorial for those however.

I have a frontlit GBA SP I had since I was a kid and it has served me well for all of these years. I don't really understand what's wrong with a frontlit screen. Are the colors better or something?

 

The colors are cleaner when backlit, frontlit ones are hazy and washed out.

 

I wonder if it would be possible to use DSi upper screen in a Gameboy Color.

PPI of the Nintendo handhelds:

GB/GBC: 82.79
GBA/GBA SP: 99.46
GBA Micro: 144.22
DS/DSlite: 106.67
DSi: 98.46
DSi XL: 76.19
2DS/3DS: 132
3DS XL/New 3DS XL: 96
New 3DS: 120.23

The PPI closest to the original GBC is the PPI from DSi. But the resolution is different, there would have to be some kind of processor to show the produced image in the middle of the screen. Even then there would be black borders around the image, albeit smaller than in the case of the GBA screen.

 

World-shatteringly so, backlit GBA SP curb stomps the front light. If you've ever used anything resembling a DS or iPhone you have a good idea already.

 

I agree. It's like night and day. The difference in the picture quality is drastic.

 

Ah ok. I've never seen a AGS-101 other than brand new on a store shelf so I never got the chance to really see the difference for myself. I have a DS lite from when I was a kid and I forget it's backwards compatible sometimes. I really only used the GBA slot for Pokemon transfers and rarely for playing games. If I wanted to play GBA games, I always played them on my AGS-001. Nowadays, I just use the Game Boy Player but every now and then, I will take my GBA SP with me and play it, but not my DS lite, since I upgraded to the DSi and then the 3DS XL long ago. Screens are getting bad on my DS lite. I'm not sure on how to fix it.

With that being said...

If it runs off of the GBC's power circuitry and the proper connections are made, it should work. I'd say go for it and see if it works.

I just measured the screen of my DSi and my old broken GBC and the DSi's screen alone is as wide as the GBC itself! Expect to have trouble fitting it in and you will have to remove the face plate on the GBC.

Maybe a small FPGA could fix the resolution issue? Not sure if you could get one for cheap that can display it in it's original resolution on that screen.

 

That's what I'm afraid of. Even if the "transplantation" should work the DSi screen may just be too large for a GBC.

 

I could try different approach. Do you know someone who has good knowledge of the GBA hardware?

 

I have doubts that a GBC would drive a DS display, you can use a 001 GBA front light, turned 90 degrees to light a GBC, with some case modding, I'm yet to see a better solution.

The GB-BOY Color is a cheap easy solution though I seem to remember an AG member reviewing it as somewhat mediocre.

 

It is medicore. It is not 100% match to real system, the sound is off and display has slightly different pixel aspect ratio so the GBC game may look a bit stretched compared to original GBC. the GB clones are fine for quick playing on the road where quality is not important.