ROM Hack GBA Fire Red (sword and shield) Romhack fails to load title screen on flash cart, most emulators...except mGBA. Why?

Phaneropterinae

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Hi,


TLDR: Sword and Shield GBA demake only can get to title screen with mGBA, curious as to why? There is another version of the rom in a different language the works fine, can I use that to fix it?


I've been working on a project of creating rom hack cartridges. I've modded some of my old GBAs and wanted to play some new colorful games. Naturally Pokemon Rom hacks were some of my first choices. I've been successful for the most part with this project, I have about 9-10 working carts with some original art and matching colorful cartridge shell.

Most recently I discovered the Sword and Shield GBA demake has been translated to english. I tested the rom initially using mGBA and it worked perfectly. So I went ahead and designed some label art and ordered a shell. All I needed to do was to get the ROM working on a cart. I've been using cheap repro carts to make my carts. When I went to put the rom on the cart, I was first met with a header bug. Nbd, I used GBATA to patch it out. After that is where I run into the title of this post. The intro scene will play, but when it ends or a button is pressed, the title screen does not appear and instead it black screens. When running the unpatched rom in VBA, same issue appears.

I wanted to see if I could debug this, so i tried to run it in the no$gba debugger. I was met with the same issue, and a warning, "CPU- Bad Operation: Undefined Opcode".
1702237942496.png


These results of a totally functional rom on mGBA, but the same error on two emulators and on hardware was baffling to me. This leads me to my questions:

What is mGBA doing differently that it is able to progress to the title screen?
Does anyone know how I can patch this to work on hardware? - I can provide a .ups patch to patch to fire red for anyone who is interested in diving in.

Additionally, I reached out the "creator" and he suggested using the "ultimate" version of the rom, which actually worked but unfortunately, its in Portuguese which I do not speak.

A follow-up question, would I be able to use some of the code from the ultimate version to get the English version to work? could I patch the ultimate version to be English using the English version?

Thanks to anyone who has taken the time to read this.
 

Phaneropterinae

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Additional info from further testing: when i go into the tools of mGBA and i turn off the VBA bug comapatibility in rom hacks, I get the same bug. What exactly is this setting doing? How can I locate the code it presumably skips?
 

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