Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!cec2!news
From: jab0396@cec2.wustl.edu (John A. Breen)
Newsgroups: comp.lang.vhdl
Subject: Resolved/guarded signals
Keywords: bus resolution function, resolved, guarded
Message-ID: <1991Mar6.232208.16052@cec1.wustl.edu>
Date: 6 Mar 91 23:22:08 GMT
Organization: Washington University, St. Louis MO
Lines: 33


I've always had trouble understanding some of the distinctions between
resolved signals and guarded signals, particularly when they cross entity
boundaries.  Suppose I have an entity/architecture as follows:

    Entity E1 Is
        Port( X: Out T Bus);
    End E1;

    Architecture A1 of E1 Is
        ...
    End A1;

where T is a type that does NOT specify a bus resolution function (BRF). 
Suppose I instantiate E1 in an architecture A2, and for the actual of X I
specify a signal Y of type RT, where RT is a subtype of T that specifies a
BRF. Suppose that, in a process in A1, I explicitly assign NULL to X (and
assume this process is the only driver of X).  Now the questions:

    o   Is X's driver turned off? Since there is no BRF, this doesn't seem
to be correct; however, what does happen to the driver?

    o   In A2, is the driver of Y from E1 turned off, or does it take on
some value?  What value?

    o   How would the answers to the above questions change if X was of
type RT?

Any comments or discussion would be appreciated.
-----
John A. Breen 				|  johnb@hobbes.mdc.com
McDonnell Douglas Missile Systems Co.	|  jab0396@cec1.wustl.edu (forwarded ^)
Tel: (314)234-4341