Ugh, wait ...

Never mind that link I posted; it's for Amiga systems. Sorry. :-P

Try this link:

http://nesdev.parodius.com/

- Tricob.

"Glass Tiger for life! "

Some games are easier to hack than others; the most commonly hacked NES games are the SMB series, ZELDA series, and Ms. Pacman (Tengen). As for adding new levels to a game, it's probably more practical to replace a level than add a new one; remember that you have strict memory limits on NES ROMs; large numbers of ROMs won't even go one byte over 64K, or 64,000 bytes.

As for writing your own NES games from scratch, you might try NES-BASIC, if I have its name right. There's a little info at this link, but I don't know if the links still work. You might try searching the keyword "nesdev" in Google, too.

http://os4depot.net/index.php?function=showfile&file=development/cross/nes_dev.lha

- Tricob.