Posted By: wraggster
HorvatM has released a new, superior assembler for the V810 processor used in the Virtual Boy, called simply "Matej's V810 assembler" or "MV810ASM" in short.
It has a working expression evaluator, local labels/identifiers, can automatically assign addresses to global variables, supports VB-specific instructions, recognizes both the "$" and "r" way of referring to registers, automatically aligns everything, has clear error messages, and you can just use it in any way you want. It also has a ROM-hacking mode with which you can apply a hack to a ROM in one step, no hex editing required.
You can get it for multiple platforms on Matej's Website. For more info and discussions check out the respective forum thread.
Also check out the source to Matej's Faceball control scheme hack for a practical ROM hacking example.