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

 
Alex,

When S'Last_Value was originally defined (with the definition given in VHDL 87), it was well known that it did not track changes over a series of delta cycles. At the time, that was acceptable. If this is now considered a problem, then I agree that a new definition is required.

However, the current definition is broken, because it no longer specifies the value of S'Last_Value prior to the first change on S. The definition

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

presumes that there IS some 'last change of S' that determines which previous value of S should be returned. In the absence of any change on S, this definition does not apply. So at the very least this definition needs to be extended to say

   Result: The previous value of S, immediately before the last change of S, if any;
            otherwise the current value of S.

Regards,

Erich

| -----Original Message-----
| From: owner-vhdl-200x@eda.org
| [mailto:owner-vhdl-200x@eda.org] On Behalf Of azro@onebox.com
| Sent: Wednesday, December 15, 2004 7:58 AM
| To: vhdl-200x@eda.org
| Subject: 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
| >
| >
| >
| >
| >
| >
|
|
|
|
|
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|
Received on Wed Dec 15 06:29:33 2004