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From: pb@PUFF.WISC.EDU (PB Schechter)
Newsgroups: comp.os.vms
Subject: Submission for mod-computers-vax
Message-ID: <8704271736.AA21017@puff.wisc.edu>
Date: Mon, 27-Apr-87 13:36:12 EDT
Article-I.D.: puff.8704271736.AA21017
Posted: Mon Apr 27 13:36:12 1987
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Path: puff!pb
From: pb@puff.WISC.EDU (PB Schechter)
Newsgroups: mod.computers.vax,comp.sys.dec
Subject: Floating point faults vs. traps
Keywords: fault trap floating-point exception
Message-ID: <716@puff.WISC.EDU>
Date: 27 Apr 87 17:36:12 GMT
Organization: U of Wisconsin CS Dept
Lines: 27


According to the VAX Architecture Reference Manual, floating point exceptions
can either be traps (which occur AFTER the instruction's completion) or faults
(which occur BEFORE its completion, and leave the instruction restartable, if
the faulting condition is removed).  I'm trying to figure out what determines
whether a particular exception will be a fault or a trap, with no success
 (i.e., I can find no explanation in the ARM).  My speculation is that, in
the general case, you get a fault, and that only if the trace trap bit (#4) in
the PSL is set to you get a trap.  My reasoning is that it's nicer to be able
to restart an incorrect operation and get things right, then to just know
that things went wrong--except when you're debugging, and you want to see
EXACTLY what went wrong.  However, this is pure speculation on my part:
I have been unable to confirm it.  In addition, of all the 10 arithmetic
exceptions, only the floating point exceptions have both traps and
faults--the rest have only traps.  This seems to contradict my theory,
unless it's more difficult to avoid writing results for integer and decimal
string instructions--I don't see why it should be.

So, if anyone can enlighten me, I will be greatly appreciative.

pb@pipe.wisc.edu

PB Schechter
U. Wisconsin
Dept. Computer Sciences
1210 W. Dayton St.
Madison, Wisc. 53706