There is no way to have GBA support on the MMD. The thread you were talking about was most likely discussing how to convert between ".ds.gba" and ".nds" files, both of which are DS binaries with different extensions. The .ds.gba files have an extra header attached to the front so that GBA flash carts will accept them. Removing that header turns it into a .nds file.
However, this does not help with .gba files that are actually GBA binaries. The explanation for why GBA files are unplayable on the MMD is kinda complicated, but I'll try to explain it in simple terms. Basically, the GBA requires lightning-fast responses that CF/SD cannot provide. The Supercard and M3 get around this by copying GBA binaries to fast RAM before executing. The MMD has no such RAM, so GBA files cannot run because they'd freeze from lack of information.
The slightly more technical explanation:
The GBA is made a different way than the DS. The DS has 4 MB of RAM, so it streams content from the DS game cartridge to the RAM and executes the code from there. The GBA however only has 256 KB of RAM, not nearly enough to do things that way. Instead, code is executed directly from the cartridge itself. Because of this, the cartridge needs to be able to respond to a request for information very very quickly. This is why GBA flash cartridges were so expensive - they used expensive NOR flash, which was very slow to write to but very quick to read from. CF and SD are NAND-based flash media as opposed to NOR-based and are therefore quick to write to but relatively slow to read from, so you cannot execute GBA binaries directly from a CF/SD card.
Sorry to dash your dreams like that.
The only way for you to run GBA binaries on your DS would be a GBA emulator for the DS, which nobody has written or even started working on as far as I know.
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