code for use16 CS modes, with PE on or off. 
Stuff that needs PE a particular way, notably reflexes, goes in PE1 or PE0.

With PE on you can switch to use32. You can only flip PE in restriction
level 0, but you can't get into use16 or PE=1 without entering restriction
level 0, and then you're stuck there, so unlike the 286, you don't have to
restart the machine to leave use16.

There may be some code pertinent to use16 modes that is not in use16 code.
Initializing things like the mode-specific use16 IDTs can be done in
use32. There may also be initialization code and so on for one thing to be
initialized FROM more than one mode, BUT, Ha3sm will do most or all init
in use32 PE1.
