From: utzoo!decvax!aps
Newsgroups: net.unix-wizards
Title: Zero vector interrupts on a VAX
Article-I.D.: decvax.460
Posted: Mon Apr 11 15:09:26 1983
Received: Tue Apr 12 02:41:33 1983

Dave, I seem to recall that an NPR has priority over any of the BR's.  I
don't recall that UNIBUS devices refused to pass back BG's if they say
an NPR.  But that doesn't mean that they don't.  Note that the rules say
that an arbitrator may not issue a grant (NPG or BG?) for at least 75 ns
after the negation of SACK to ensure that the arbitrator does not
arbitrate old NPR/BR signals.  At anyrate, I understand that some
devices actually time out before they get grant back from the
arbitrator.  In the case of a VAX, that fact is reported to the CPU
during the interrupt summary read.  In PDP-11's, it is ignored by lower
levels.

As to answer your question, I don't know why the count of zvi's couldn't
be reset every now and then or better yet, note the time of a zvi and if
you get another one in some interval, then do a reset.
	aps.