Currently porting to RISC-V

[retpolanne]

Hey, this is my attempt at porting it to RISC-V :)

https://github.com/retpolanne/rust-mangopi-OS-tutorials-riscv

The challenge I'm currently facing is that, for some reason, the stack frame gets a lot of 0xff's and at some point the pc points to that.

Edit: I made the hello world work :)