[sv-cc] Value change callbacks with variable indexes -- and (not) for automatic variables

Abi, Chuck, et alii,

At Charles's suggestion, I checked with a member of the now-defunct PLI 
Task Force, David Roberts.  He recalled a long-ago but vigorous 
discussion of what a cbValueChange callback should be sensitive to.  
The task force's decision was to be consistent with the sensitivity of 
event control expressions and $monitor() arguments.  One can poke 
around the LRM for statements about what that means, but in short:

"... when you put a callback on a[i] you should get a callback when 
ever the value pointed at by a[i] changes.  This can happen because the 
value in the object index by "i" changes
   reg a[3:0];

   i = 0;
     put cb on a[i].
   now update the value of a[0].  a[0] = 1;
       a[i] just went from an X to 1.

the callback should fire when "i" changes if and only if the value that 
a[i] now points is different then what it was from the last time the 
callback fired.

...

And yes we have to handle the case when the index goes out of range.  
For that case the value is "X"; this behavior is defined by the Verilog 
HDL language."

So at this point I'm thinking of simplifying our description of what is 
and is not supported by referring to what event control statements and 
$monitor() arguments can handle and are sensitive to.

Incidentally, one side effect of this is that we can probably exclude 
automatic variables and their subelements as value change arguments, 
since the $monitor() system task does not allow them (Draft 4, 13.3.2).

I don't know yet if they can be used as event control expressions, 
though.

Regards, 
Jim V.

--------------------------------------------------------- 
James H. Vellenga                            978-262-6381 
Software Architect                     (FAX) 978-262-6636 
Cadence Design Systems, Inc.         vellenga@cadence.com 
270 Billerica Rd
Chelmsford, MA 01824-4179
"We all work with partial information." 
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