But VAXes HAVE flashing lights...

[Dave Martindale]

Actually, the lights on the 4th CPU board in tell you nothing, because they
are all partly lighted all the time when the CPU is running.  The ones
on the 3rd board are the CPU microcode address - they can tell you
whether the CPU is idle or not, but not too much else.  I believe the
uppermost light on that board tells you that the CPU is stopped waiting
for something like a memory fetch.

Even more useful are the microcode address lights on the Unibus adapter -
they show whether there is any unibus activity, and how intense it is.