Things that need to be done:

Kernel:
- D.BTLO and D.BTHI need to be set.  This is usually done in KrnP2's JmpBoot,
but the Atari port doesn't have a boot module.  We need to rethink how this will get
done  in this port as well as the CoCo port as CoCoBoot moves forward.

VTIO:
- SS.KySns needs to be implemented

dw3.sb:
- Currently branch to init serial VIRQ is commented out.  Had system reliability
troubles with it uncommented.
- NMI as clock source could be a problem for DriveWire because OP_TIME could come in
the middle of a DriveWire read or write.

Other:
- CoCoBoot needs to be ported to the Atari.