Path: utzoo!attcan!utgpu!watserv1!watdragon!rose!ccplumb
From: ccplumb@rose.waterloo.edu (Colin Plumb)
Newsgroups: comp.sys.transputer
Subject: Re: Help: Error Pin on T800 Transputer
Keywords: transputer, architecture
Message-ID: <19381@watdragon.waterloo.edu>
Date: 18 Dec 89 16:24:54 GMT
References: <5979@ubc-cs.UUCP>
Sender: daemon@watdragon.waterloo.edu
Reply-To: ccplumb@rose.waterloo.edu (Colin Plumb)
Organization: U. of Waterloo, Ontario
Lines: 27

I'm rather down on the error flag, but you can certainly connect it to
another transputer's event input pin.  Inmos describe that
configuration in some of their literature.

When I said that the error flag can be ignored "if the hardware is
agreeable," I meant if the error pin isn't connected to anything
important or non-maskable.  You seem to be considering making
"disagreeable" :-) hardware.  This is still possible, just requires
more careful code generation, since the error is one more output of an
instruction that you have to be concerned about.  Most of the normal
arithmetic operations set the error flag on signed overflow, so while
they can be used for unsigned arithmetic, the error flag will quickly
get set.  You have to use sum (or bsub) instead of add, diff instead of
sub, etc. if you want to use unsigned arithmetic.

Urg... that's not particularly clear.  Unfortunetely, I'll last read
mail and news tomorrow (Tue, 19 Dec) morning before going home for
Christmas and New Year's.  So if I've confused you, write quickly or be
patient!

One note about the event input pin: being edge-sensitive, many
commercial interrupt controllers will trigger it multiple times, as
their outputs are designed for level-sensitive processors and not
particularly clean.  The Am9519A, in particular, has *big* glitches on
its ourput, much longer than the documented 100ns.
-- 
	-Colin