Re: [Iverilog-devel] assign/deassign and force/release of bit/part selects.

[Stephen W. <st...@ic...>]

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Cary R. wrote:
> I'm working on the implementation for assign/deassign of bit and part
> selects and decided to see what the standard had to say. Very interesting!
> 
> For reference this is what it says about assign (section 9.3 page 125
> 1364-2001):
> 
> The left-hand side of the assignment in the assign statement shall be a
> variable reference or a concatenation of variables. It shall not be a
> memory word (array reference) or a bit-select or a part-select of a
> variable.

It says the same thing. I understand why they disallow memory words,
and we should stick with that. I'm surprised about the bit/part
select exclusion, but given the trouble *we* are having with it,
I can understand why it is there;-)

> The way I read this is that an assign only works on a register/etc. type
> not a wire (net). The standard say we shouldn't allow an assign to an
> array word, but we don't distinguish between individual array words and a
> normal variable. Bit and part selects are not allowed, but I'm working on
> implementing them.
> 
> It says basically the same think for force except it allows force to work
> on either variables or nets. Though it is missing the array word
> exclusion. 

The 2005 spec is explicit that it should not allow memory words
or array references. I think we *can* allow array references if
the address expression is constant. I think that would be more
interesting then adding assign/deassign to bit/part selects.

As for accepting assign/deassign of bit/part select, I guess we
can add support to them if it is not too hard. We may want to
think about adding a warning type "extensions" that would flag
this sort of thing so that people can vet their code for portability.

- --
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."
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