Does anyone here know how to figure out what assembly language games for undocumented consoles are written in?

There is a mini-list of consoles and their respective assembly languages in The Definitive Guide to ROM Hacking for Beginners, but it doesn't have certain consoles such as the PC-FX.  Furthermore, even with the consoles listed there, the assembly languages for each of them was unknown at one time, and it was up to anyone with the right skills to figure out the respective assembly language.

So, how does one figure out which assembly language games are written in for any given console?  Even if it's not the PC-FX?

So, since the PC-FX's CPU is called the NEC V810 (according to Wikipedia), that means that the respective assembly language is called either 810 or V810.  Right?

http://www.goliathindustries.com/vb/download/cpu/U10082EJ1V0UM00.pdf

Specification sheet. Google's your best friend. Features a quick introduction to the registers and has a full opcode listing. Now you need to know whether the PC-FX uses a derivative processor (the gameboy for instance uses a z80 variant that has some instructions added and others left out) or just this standard processor. It might be useful to google for additional opcode listings that specify them in a simpler way, though.

cYa,

Tauwasser

Well, there's the matter of actually getting code into the ROM.

Some assemblers like xkas expedite the process by letting you compile code directly into a ROM. But if you're working with anything other than the SNES, you'll probably have to insert your compiled code manually via a hex editor. There's also the issue of actually fitting it into the ROM: on occasion you can simply overwrite an old routine with a new one (if the new one's code is smaller), but other times you'll have to find some way to wedge it into blank space, and on systems that employ bank swapping, this isn't always easy. Platforms which use a file system add the wrinkle of fitting your code into memory: I had to resort to a bit of trickery in order to add a code hack to Death Note: The Kira Game.

And assuming you do have to find some place to fit your code, it still won't exactly execute itself. You need to hijack the game's code so it jumps to your routine. Sometimes this is as easy as finding the jump for the routine you want to replace and changing it so that it points to your code instead; other times, you'll have to reposition pieces of the original code in order to insert the jump.

Most likely yes. In my experience I have to do a bit of code relocation in all but the most minuscule hacks (such as changing the hard-coded width of a sprite). Once you've done it a few times it will be easy as cake. My general technique is this:

1) Find some free space in the ROM. Say I found some at 1234 that is big enough to fit my code.
2) Change an instruction of the original routine to JMP 1234.
3) At the end of my routine at 1234 it will jump back.

Thanks guys. 

Hopefully, I'll be able to figure out sprite hacking on my own when I get around to learning the V810 assembly language.  But here's one more thing I'd like to ask:

Does anyone here know how to convert home-made voice clips into hexadecimal?  For instance, if I wanted to overwrite the audio line "Urusaiwane!" with "Fussy, aren't you!"  I'm assuming that once it's converted from an audio file into hexadecimal, I might be able to use it to overwrite the original line in a hex editor, but I don't know how to even convert it to hex, let alone port it.  Could someone fill me in on that, please?

Also, am I correct to assume that the music, sound effects, and dialogue in FMVs are all stored in separate bytes?  If that's true, then hopefully, I'll be able to overwrite the original dialogue without tampering with anything else within any given FMV.  Right?

Strictly speaking, any binary file (or any file at all, really) is already in hexadecimal.  Hex editors are used to view the numerical value of each byte in a given file.  This is as opposed to standard file editors (like Notepad) which will convert each byte into its ASCII value and display that instead (or convert groups of bytes into their Unicode values, etc).  What you really need to know is (a) what audio format the game is using, (b) how do you convert your existing audio file into that format, and (c) how do you replace the old audio data with the new audio data.

Two more things...

If a game's graphics, script or whatever else is compressed, I understand that an assembly tool is required to decompress whatever I might be trying to change.  What steps are involved in decompressing the data in question before the game becomes compatible with a graphics editor, script dumper, etc.?  Or does compressed graphics/scripts/etc. mean that I'll have to modify data manually?

Also, when I go into the Utilities section of this site, there are no tools that are exclusive to the PC-FX, although there are tools that say "Multiple" or "NA" in the "Platform" column.  What's the difference between "Multiple" and "NA"?  Will anything labeled as either be compatible with games for undocumented consoles such as the PC-FX?