Crash followed by green db> prompt

["Thomas Mueller" <mueller6724%bellsouth.net@localhost>]

Running NetBSD 5.1-RC3 on i386, editing a file, the kernel configuration, 
with vi, I had a sudden crash followed by green 
db>
prompt, in contrast to the usual white (light gray?) on black.

Point of this question is what can I do from that prompt.  ls, mount, help
didn't work.  Best thing I could think of was type reboot; that worked, doing
just what you'd expect for reboot.  But what else could I do, where is it
documented?  I don't know whether this is specific to just a few architectures,
such as i386 and amd64, or if it is for all NetBSD-supported architectures.

Maybe there is some known instability in NetBSD 5.1_RC3, judging from the long
time elapsed since 5.1_RC3 with no release or RC4?  But my main question is
what I can do at the post-crash db> prompt, which looks like something that
would not be just for -current or release candidates.

Tom