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I've been trying to hack Zelda: Wand of Gamelon. (Discussion of things like this is allowed, right? Because it's not a ROM request.) Finding data is no problem; I've located many raw text banks. But whenever you change a single byte of the ROM, it refuses to load and just gives a black screen, then switches back to the CD-i menu.

Would anyone know how I can stop this from happening?

You say 'ROM' but you're really editing a disc image file (ISO), right?
And using it with a real CD-i player, not CD-i Emulator?

In this case editing bytes won't work because every sector contains error correction (ECC) and detection (EDC) bytes (like CRC but different). If you change data bytes, the ECC and EDC bytes need to be updated; the player checks them when reading the disc.

I don't know if there are tools to update the ECC/EDC bytes for you; this is probably not something done very often.

If you're using CD-i Emulator, it doesn't check ECC/EDC and editing data bytes without updating ECC/EDC should work.

Yeah, I'm editing from the ISO, but I'm using CD-i Emulator, not an actual CD-i. I'm not editing the ASM segments, only data segments (e.g. "Start New Game", things like that).

There is also another internal checksum on code modules; this is checked by the OS-9 operating system. The algorithm is described somewhere in the OS-9 documentation; check the F$Crc system call.

Okay, I checked that F$CRC thing out...didn't really understand it. What would I be looking for here, and how would I disable this kind of checksum system (it's easier than updating it every time I make a minor edit)?

Also, what format would the FMVs be?
Sorry I'm asking so many questions.