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: ownervhdl200x@eda.org
 [mailto:ownervhdl200x@eda.org] On Behalf Of azro@onebox.com
 Sent: Wednesday, December 15, 2004 7:58 AM
 To: vhdl200x@eda.org
 Subject: FW: RE: RE: [vhdl200x] VHDL LRM issue re: S'Last_Value

 Steve:

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

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

 Others might also want to read the explanation.

 Regards,

 Alex Z


 Alex Zamfirescu
 azro@onebox.com  email
 (877) 3320676  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: [vhdl200x] 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) 3320676  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: [vhdl200x] 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: ownervhdl200x@eda.org
 > [mailto:ownervhdl200x@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; vhdl200x@eda.org
 > Subject: RE: [vhdl200x] 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) 3320676  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>;"VHDL200x"
 > <vhdl200x@eda.org>
 > Subject: [vhdl200x] 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 10761987 LRM contains the following definition for
 > S'Last_Value (on page 148):
 >
 > 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 10761993 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 10762002 LRM contains the same definition of
 > S'Last_Value (on page 198) as appears in the VHDL 10761993 LRM.
 >
 > Recommendation:
 >
 > Restore the definition of S'LAST_VALUE to the form that
 > appeared in the VHDL 10761987 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
 >
 >
 >
 >
 >
 >








Received on Wed Dec 15 06:29:33 2004
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