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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Question about 'valid
Sender: bobduff@world.std.com (Robert A Duff)
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Date: Thu, 29 Mar 2001 23:56:58 GMT
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Date: 2001-03-29T23:56:58+00:00
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Peter Amey <pna@praxis-cs.co.uk> writes:

> Given X : T;
> 
> If X'Valid is true and X is scalar is it always true that:
> 
> X in T;
> X >= T'First;
> X <= T'Last;

Yes.

> If the answer is yes then is it still true if T is an enumeration type
> with a representation clause which leads to holes in its representation.

Yes.  But I would be a little bit wary of compiler bugs in that area.

> I am currently very interested in validating aribitrary bit patterns
> that arrive via memory mapped ports and want to really understand the
> role of 'valid in these cases.

I would avoid hole-y enums for this purpose, and just use integer types
that happen to have no extra values (eg "mod 2**32" or whatever).

- Bob