Back when I was looking into multi-headed SPARC boxes (in the
early 1990s ...) the PROM only initialized the settings in the
display card for the (single) monitor set up as the output
device.

The monitor's capabilities show up at one of the bw2/cg3/cg6
registers ("monitor sense"), and from that, *some*thing has to
set all the parameters (when to set & clear horizontal and
vertical blanking, sync, etc.).  SunOS had code to do it in
each driver.  What I was going to do instead was write a user
program that would read the monitor sense bits, read a file in
/etc (monitorcap?), and correlate the one with the other to
program the Brooktree registers.  This would (a) keep the code
out of the kernel, and (b) allow people to tweak their settings
if they wanted.

Of course, I never got around to writing the user program....

Chris