I hadn't seen tow-boot. Was familiar with u-boot though.  This is interesting: https://wiki.pine64.org/wiki/PineTab-V

I have a SiFive HiFive Unmatched running ubuntu. I tried to write low-level code to do what u-boot should do on that hardware (wind up the PLLs, flip some switches, etc, the stuff the HW manual says must be done at power on) and then print something to the screen.  But it just printed garbage to the screen and I never figured out why my code wasn't working... and then I discovered nostr and my life changed.