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

11.5.1 says,

"Part-selects that are partially out of range shall, when read, return x for the bits that are out of range and shall, when written, only affect the bits that are in range."

Shalom

> -----Original Message-----
> From: Jonathan Bromley [mailto:jonathanbromley@ymail.com]
> Sent: Wednesday, March 23, 2011 2:48 PM
> To: Bresticker, Shalom; Steven Sharp; Gordon Vreugdenhil; sv-bc
> Subject: 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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