$4201 " Programmable I/O port (out-port) [WRIO] "
$4213 " Programmable I/O port (in-port) [RDIO] "

That's all the info I can find on these two, unless the book I ordered explains this, I wont know what it is. I know I should wait for it, but I can't sit here and do nothing until the book arrives, I'm the type of person that has to do something, I cant wait - I gotta find out and fiddle around with some basic rom coding. If it explains it in the book, then it will reinforce what I learned and it will be easier to remember, if it doesn't then this is the only way I'll ever find out, cuz I can't find anything on the web that explains it. I was fiddling around with my snes initializatio rom code, and when I put this code it crashes zsnes. I didn't come up with it, I copy/pasted it from another similiar initialization file.

LDA #$FF
STA $4201 ; Programmable I/O port

It crashes unless I put this command

LDA #$80

Which doesnt make any sense because it needs (and is followed by) this instruction to work?

STA $2115

Which is the vram port. I'm guessing somehow the two are connected, is it something to do with the monitor output? bleh

anyone have any idea? I could post my whole source file if you want, but it is rather long and unoptimized, all it does is initialize snes with cleared registers and clear the vram. It's not my own, I just looked at two other similiar source files and put the two together with some changes. I'm just doing it so I can see how the registers work/crosschecking from yoshi's snes documentation, so it wouldn't be in any real coding I do.