I have no idea how this stuff is *implemented* in NetBSD, but
there is a logical difference between:

	there is memory from A to B and from C to D

and:

	there is memory from A to D, but from B to C must
	not get used by the kernel

This difference might be important to (for instance) /dev/mem,
which should permit access to addresses in [B..C) in the second
case, but not in the first case.

The case of Sun-3 video RAM is much more arguable than that
of SPARCs-with-boot-proms-reserving-space, though.  The video
RAM *is* RAM, but is rather different from ordinary RAM, while
the RAM taken up by the boot PROM is perfectly ordinary, just
`not available for general kernel/user use'.

Chris