To fix this, do the following:
* Remove the incompatible board or boards.
* If you still have your video card installed (assuming it's not the
incompatible card), start up from a 6.0.x floppy and you will get a
dialog prompting you to change to 24-bit addressing.
- OR -
* If you don't have your video card, start up from your System 7 and
reset the Parameter RAM by holding down Command-Option-p-r at start up.
These procedures will reset the addressing to 24-bit mode.