Re: [sv-bc] 4-state or 2-state expression types

It seems, then, that we can make the problem go away - and completely answer
Gord's question that started this thread - by specifying that the result of an
out-of-range access on a 2-state packed object is a 2-state '0. That's
compatible with the de facto behaviour of three major tools, with the exception
of one rather obscure corner case (the tool that treats statically-known
out-of-range accesses differently). It would be a straightforward update of the
text in 11.5.

Any strong objections?

One final wrinkle: Suppose I do an invalid part-select that overlaps the valid
range. Should the entire result be '0/'x, as the LRM suggests? Or should the
bits that are within range be given their "correct" value, as if they had been
picked one-by-one using a 'for' loop? Example:

   logic [7:0] B = 'h55;
   logic [7:0] Slice = B[11:4]; // Should Slice be 8'hx5 or 8'hxx?

Thanks

Jonathan

----- Original Message ----
> From: "Bresticker, Shalom" <shalom.bresticker@intel.com>
> To: Jonathan Bromley <jonathanbromley@ymail.com>; Steven Sharp
><sharp@cadence.com>; Gordon Vreugdenhil <gordonv@model.com>; sv-bc
><sv-bc@eda.org>
> Sent: Wed, 23 March, 2011 12:19:56
> Subject: RE: [sv-bc] 4-state or 2-state expression types
>
> Also, Table 7-1, which specifies values to be returned from a non-existent
>array entry, already specifies that reading a non-existent 2-state element
>returns '0. More generally, a value is always returned that is in the value set
>of the corresponding data type. I find it questionable that 2-state integral
>types should be exceptions.
>
> Shalom
>
> > -----Original Message-----
> > From: Jonathan Bromley [mailto:jonathanbromley@ymail.com]
> > Sent: Wednesday, March 23, 2011 1:46 PM
> > To: Bresticker, Shalom; Steven Sharp; Gordon Vreugdenhil; sv-bc
> > Subject: Re: [sv-bc] 4-state or 2-state expression types
> >
> > I spoke a little too soon...
> >
> >
> >
> > > The two simulators I can try right now disagree about this. Doing
> > an
> > > out-of-range select on a 2-state vector, one yields 4-state 1'bx,
> > the other
> > > 2-state 1'b0.
> >
> > The third big-name simulator also yields a 2-state result.
> >
> > In fact the simulator that yielded 4-state 1'bx did so only when the
> > subscript
> > was statically known to be out of bounds (and it gave me an
> > elaboration-time
> > warning for that too). An out-of-bounds subscript computed at runtime
> > gave
> > 2-state 1'b0, just like the other simulator. I don't know whether that
> > should
> > affect BC's thinking...
> >
> > Jonathan Bromley
> >
> >
> >
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