[vhdl-200x-ft] FT-12: comments on reading out-mode parameters

Folks,

I was reviewing the notes from the 26-Jul-04 telecon relating to FT-12
(allowing reading of out-mode interface objects). The notes indicate that
participants wanted to allow reading out-mode signal parameters and ports,
but not to allow reading of out-mode variable parameters.

I'd prefer we uniformly allow reading of out-mode objects, as proposed in
the current FT-12 document. This would mirror the change made in Ada-95. I
offer the following comments from the Ada-95 Rationale:

  6.1.1 Out Parameters

  In Ada 83 a formal parameter of mode out could not be read, even after
  being initialized within the procedure. This forced certain algorithms
  to include a local variable just to accumulate a result and which was then
  assigned to the out parameter. Introducing such a local variable is error
  prone, because the final assignment may be mistakenly omitted.

  Similarly, if an out parameter is passed to a second procedure to be
filled
  in, the value returned cannot be checked prior to returning from the first
  procedure.

  For Ada 95, we have removed the restrictions on the use of out parameters.
  Specifying that a formal parameter is of mode out indicates that the
caller
  need not initialize it prior to the call. However, within the procedure,
  once the parameter has been initialized, it may be read and updated like
  any other variable. As with a normal variable, it is an error to depend on
  the value of an out parameter prior to its being initialized.

  ...

I think the same arguments apply in the case of VHDL. As testbenches become
more complex, code in subprograms will become more and more like regular
software code. I see no reason for us to assume that testbench developers
will be immune from the pitfalls mentioned above for software developers.

End $0.02.

Cheers,

PA

--
Dr. Peter J. Ashenden                        peter@ashenden.com.au
Ashenden Designs Pty. Ltd.                   www.ashenden.com.au
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