------------------------------ On Thu, Feb 28, 2013 4:21 PM PST David Riley wrote: I would have guessed extension roms contain raw independent code. Largely just writing values to port addtesses Which lead me to ask what a video device driver is/does. Presumably when you type Mode co,80 iirc, or screen 1 in basic, you're assigning different values to ports ie fiddling with say the 6845's registers, thereby creating another screen on the fly. Unless I'm wrong. But if not, isn't that kind of what a dd does? There's bound to be more then that going on. But code is code, regardless of what stage of the game you're in, start up, boot up, os fully installed. The os only has as much control as you allow it. You communicate with dos, but s/w at least can manipulate the h/w. Which is why s/w has too much power under dos. When dos transfers control to a bunch of code, it's boss.