Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!rpi!bu.edu!inmet!davidb
From: davidb@inmet.inmet.com
Newsgroups: comp.lang.vhdl
Subject: Re: resolved/guarded signals
Message-ID: <381900010@inmet>
Date: 14 Mar 91 15:32:00 GMT
References: <223755@<1991Mar13>
Lines: 20
Nf-ID: #R:<1991Mar13:223755:inmet:381900010:000:924
Nf-From: inmet.inmet.com!davidb    Mar 14 10:32:00 1991


/* Written  5:37 pm  Mar 13, 1991 by jab0396@cec2.wustl.edu */

>Why are only guarded signals allowed to be assigned NULL explicitly
>(as opposed to resolved signals in general)?

The normal answer to any "Why?" question concerning the LRM is,
"Because!".  This one has a more rational answer; the use of the null
waveform directs that the driver contining that null waveform not
provide a value to the resolution function used in determining the
value of the signal.  Some indication must be given concerning what to
do when all drivers "turn off".  This is provided by the two keywords,
"register" and "bus" that identify the signal as a guarded signal.
When "register" is used, the resolution function is not called with
zero drivers, and the signal retains its value.  When "bus" is used,
the resolution function is called with an empty array (i.e., an array
with no values).

						Dave Barton
						barton@i2wash.com