AppendFile #3
Description
I know you said not to look at a disassembly but I just thought I'd let you know that AppendFile has one line of code:
LDA #255
So that's another command you can cross off the list :P
I believe from there the program pointer starts looking at the code for WriteFile, so AppendFile is just an expansion of WriteFile, it seems.
You know, if you're feeling stuck, you can use the FDS BIOS disassembly on NESDev as a reference for this. Like, not copying the code directly but looking at it to know what it does. Considering the amount of code you replaced on your own, using modified code snippets of the original assembly shouldn't be much of a problem, as long as you aren't using any graphics from the original FDS BIOS. For example, you'll probably need to completely rewrite the startup code.
Sorry I couldn't help much on the original code front, the fact that reverse-engineering the FDS BIOS is incredibly hard combined with the fact that I'm not too experienced with 6502 Assembly makes it hard to contribute for me. Huge props to you for recreating so many of these functions. I hope you're able to successfully finish this project someday.