[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
Received on Tue Dec 14 12:16:25 2004