RE: [vhdlsynth] Re: [vhdl-200x-ft] Bit size rules

> -----Original Message-----
> From: Marcus Harnisch [mailto:marcus.harnisch@gmx.net]
> Sent: 10 July 2004 19:55
> To: Jonathan Bromley
> Cc: vhdl-200x-ft@eda.org; vhdlsynth@vhdl.org
> Subject: Re: [vhdlsynth] Re: [vhdl-200x-ft] Bit size rules

> >PROPOSAL
> >~~~~~~~~
> >Require that any fixed-point object or subtype have
> >a descending array range.
> >
> >IMPLEMENTATION SUGGESTION (2)
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >Modify the base language so that unconstrained
> >vector types can be declared to have a range that
> >is required to be ascending or descending.

> Could that perhaps be addressed by allowing to set the
> 'ascending attribute?

I don't much like that idea myself - it sounds like just a
slightly different way of doing

  assert [not] my_array'ascending

and the idea of attempting to set something that's a static
property of an object doesn't really appeal.

As I said, I don't really want to get bogged down in syntax
proposals right now, since they're not central to the basic
idea. Further down the line we could easily imagine various
ways to do the necessary syntax. One possibility, which
requires no new syntax but does require a significant change
in language definition, is to permit just one subtype
constraint to be applied to an existing constrained array
type, so that

  type ascending_bitvector is array (0 to 10000) of bit;

would have the effect of setting the direction and
absolute limits on subtypes of ascending_bitvector, so that
a subsequent

  subtype ascending_byte is ascending_bitvector(0 to 7);

would be legal.

Regards

-- 
Jonathan Bromley, Consultant
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