Custom OS

[z4v4l]

Also, why would it matter if my OS was written or not? That has nothing to do with the subject matter.
Anyways, I thought uboot was stored on the board in the ROM. Was I incorrect in assuming that?

It has. Because if one "has written OS", he/she probably already knows about the platform far beyond that trivial thing as "what partition to boot from has to be there". Because you asked this question, I was wondering.
and this sentence is just brilliant, to not wonder:

and I was curious if it's possible to run a custom OS I've written on it

Don't get pissed off, it was a friendly irony, I wish you success with your OS, you "have written". OSDev is my passion too. That was actually why I reacted. Btw, I also "have written" firmware for Pine64, UEFI compliant (in fact I am sitting and trying to implement AllocatePages() Boot Service routine right now), so you might consider using it for your OS. In future, when our fantasies will mature into reality a little.