You can modify GBA roms using a Hex editor, but it isn't easy to use.
poketronic is a program you can use to edit Pokemon gba roms. there are more tools like this that do different things.
To get ds hombrew you need a flash cart such as R4, M3, CycloDS, or (if you also want to use gba roms) Neoflash. To get Homebrews just use a searh engine such as google.com.
Yes and No... Emulators can not use wifi outside the computer but it can hook up to another game by opening another link vba or no$gba
Yes you can but you need a GBA Player cable that hooks up to the Gamecube see there are 2 cables that let you use your GBA and give stuff or play stuff on the gamecube there is the GBA to GCN cable and the GBA player cable get the player cable and you will be able to play GBA games on the Gamecube.
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No a N64 keyboard does not exist. You can play N64 games online using an emulator which you then use a keyboard to control, but there is no attachment keyboard for the N64 system. .j.
It more depends on the rom, I use No$GBA and i have found roms that work and some that don't. Though i recommend actually getting the game cause its much harder to use the capture system with your mouse
A DS's Slot-2 (GBA slot) is unable to use the DS's wireless capabilities, even if it's a GBA game which could use the GBA wireless adaptors. You can only use the online portions of a GBA game using an actual GBA and a GBA wireless adaptor.
No although it's impossible to use the GBA for the game. You can use the GBA to play the pokemon games that are compatible with Pokemon Stadium
No, the GBA slot of a DS is unable to use the wireless capabilities of the DS. You cannot link two DS's together to use the GBA part of the consoles. To do so, you must use actual GBAs and a link cable/GBA wireless adaptor.