FW: RE: RE: [vhdl-200x] VHDL LRM issue re: S'Last_Value

Steve:

I forwarded this via reflector because I got this
delivery failure.

SBailey@model.com
Delivery failed
550-The address specified
550-in your email is not a recipient recognized by this
550-E-Mail system....

Others might also want to read the explanation.

Regards,

Alex Z

Alex Zamfirescu
azro@onebox.com - email
(877) 332-0676 - voicemail/fax

-----Original Message-----
From: azro@onebox.com
Sent: Wed, 15 Dec 2004 07:44:31 -0500
To: SBailey@model.com;azro@onebox.com
Subject: RE: RE: [vhdl-200x] VHDL LRM issue re: S'Last_Value

Steve:

I did not claim that there are deltas before the first one at time
zero. What I meant was that Erich's proposal to go back to the
'87 definition should not be followed, because the '87 definition
was wrong. It was wrong in not providing for the case where
there is a change in a particular delta that is two or more deltas
behind now. Going back to that definition (based on whatever reason
to fix the first delta of the time zero) would bring back the problem
for these other cases, and other problems with the composite types.

Here is a simple example

.
     Signal S : integer := 0;
begin
     S <= 100 after 5 ns;
wait on S;
     -- Here S is 100
.
    S <= 101;
wait;
    -- Here S is 101;
    S <= 102;
wait;
    -- Here S is 102
wait;
    -- Here S is also 102
wait;
    --S'last_value should be 101 here,
    --but S'Delayed(0 ns) is 102, and S'Delayed(5 ns) is 100.
----------
Besides the fact that the '87 explanation

      "For a scalar signal S, S'LAST_VALUE = S'DELAYED(T) where T >= 0ns is the
      smallest value such that S'STABLE(T) is FALSE. If no such T exists, then
      S'LAST_VALUE is equal to S."

contradicts the intended well expressed definition

      "Result: The previous value of S, immediately before the last change of
S."

in cases like the one I described, there is a second problem with that (the '87 def)
related to:

      "For a composite signal S, S'LAST_VALUE is equal to the aggregate of the
previous values of each element of S."

Using this definition S'LAST_value might take a value that the composite signal
S never got.
Does this require explanations too?

Anyway, please do not revert last_value definition to its *last_value* (that before
its last change in '93),.

Regards,

Alex Z
 

-- 
Alex Zamfirescu
azro@onebox.com - email
(877) 332-0676 - voicemail/fax
-----Original Message-----
From:     Bailey, Stephen <SBailey@model.com>
Sent:     Tue, 14 Dec 2004 22:14:49 -0800
To:       <azro@onebox.com>
Subject:  RE: [vhdl-200x] VHDL LRM issue re: S'Last_Value
Alex,
Maybe I'm missing something, but how can there be one or two deltas
before time 0?
-Steve Bailey 
> -----Original Message-----
> From: owner-vhdl-200x@eda.org 
> [mailto:owner-vhdl-200x@eda.org] On Behalf Of azro@onebox.com
> Sent: Tuesday, December 14, 2004 11:09 PM
> To: erichm@cadence.com; Bailey, Stephen
> Cc: Jim@synthworks.com; vhdl-200x@eda.org
> Subject: RE: [vhdl-200x] VHDL LRM issue re: S'Last_Value
> 
> Erich:
> 
> The definition from 1987 was not correct because it did not 
> provide for the case where changes occurred two or more deltas before.
> Restoring to '87 (as you recommend) is not the correct solution.
> 
> Regards,
> 
> Alex Z
> --
> Alex Zamfirescu
> azro@onebox.com - email
> (877) 332-0676 - voicemail/fax
> 
> 
> 
> 
> -----Original Message-----
> From:     Erich Marschner <erichm@cadence.com>
> Sent:     Tue, 14 Dec 2004 12:16:21 -0800
> To:       "Bailey, Stephen" <SBailey@model.com>
> Cc:       "Jim Lewis" <Jim@synthworks.com>;"VHDL-200x" 
> <vhdl-200x@eda.org>
> Subject:  [vhdl-200x] VHDL LRM issue re: S'Last_Value
> 
> 
> Steve,
> 
> This is a reminder regarding the VHDL LRM issue that we noted 
> today during the IEEE 1850 Extensions SC meeting. 
> 
> Issue: Definition of S'Last_Value was apparently broken in 1993
> 
> Summary:
>   - VHDL 87 defines the value of S'Last_Value at time 0 (and 
> prior to the first change on S) to be equal to S.
>   - VHDL 93 does not appear to define the value of 
> S'Last_Value at time 0 / prior to the first change on S.
>   - VHDL 2002 contains the same definition as in VHDL 93
> 
> Details:
> 
> The VHDL 1076-1987 LRM contains the following definition for 
> S'Last_Value (on page 14-8):
> 
>   S'LAST_VALUE
>       Kind: Function
>       Prefix: Any signal denoted by the static signal name S.
>       Result Type: The base type of S.
>       Result:  The previous value of S, immediately before 
> the last change of S.  Specifically:
> 
>       For a scalar signal S, S'LAST_VALUE = S'DELAYED(T) 
> where T >= 0ns is the 
>       smallest value such that S'STABLE(T) is FALSE.  If no 
> such T exists, then
>       S'LAST_VALUE is equal to S.
> 
>       For a composite signal S, S'LAST_VALUE is equal to the 
> aggregate of the previous
>       values of each element of S.
> 
> Note that this defines the value of S'Last_Value at time 0, 
> by saying that if no previous change on S occurred (i.e., no 
> T>=0ns exists for which S'STABLE(T) = False), then 
> S'Last_Value is equal to S.  This applies at time 0, since 
> all signals are presumed to be stable at time 0, and to have 
> had their initial values for an infinite time into the past 
> (see 12.6.4, The simulation cycle, first bullet of 
> initialization phase), and furthermore continues to apply up 
> until the first change on S.
> 
> The VHDL 1076-1993 LRM contains the following definition for 
> S'Last_Value (on page 188):
> 
>   S'LAST_VALUE
>       Kind: Function
>       Prefix: Any signal denoted by the static signal name S.
>       Result Type: The base type of S.
>       Result:  The previous value of S, immediately before 
> the last change of S. 
> 
> Note that the specific details were apparently deleted, so 
> the new definition no longer addresses the issue of the value 
> at time zero (and up until the first change), nor does it 
> address the meaning of S'Last_Value for a composite signal S. 
>  It may be that it is unnecessary to define S'Last_Value 
> separately for scalar and composite signals, but it is 
> clearly important to define the value of S'Last_Value at time 
> 0 (and prior to the first change on S).
> 
> The VHDL 1076-2002 LRM contains the same definition of 
> S'Last_Value (on page 198) as appears in the VHDL 1076-1993 LRM.
> 
> Recommendation:
> 
> Restore the definition of S'LAST_VALUE to the form that 
> appeared in the VHDL 1076-1987 LRM.
> 
> Regards,
> 
> Erich
> 
> -------------------------------------------
> Erich Marschner, Cadence Design Systems
> Senior Architect, Advanced Verification
> Phone: +1 410 750 6995   Email: erichm@cadence.com
> Cell:    +1 410 294 2599   Email: erichm@comcast.net 
> 
> 
> 
> 
> 
>