"Register bit twiddling." Setting all the modes that all their various cards can operate in, with the associated code for sending the bit updates over the connection bus. Tedious stuff that's very prone to copy-paste errors if written by hand.
At some point you have to take AMDs word for it that these codes = this functionality, but if the right graphics come out then it can't be so wrong.
Compiling has never been the hard part. The challenge is making it through the entire configuration menu system before succumbing to the urge to gouge your own eyes out with blunt sticks.
Once that's done, kick off make take a long break; it'll be compiled by the time you get back to it.
I hear build times are getting longer with the Rust parts, though, so do it soon before you need mainframe access to get a compile within your lifetime.