Re: [vhdl-200x] Update to proposal for arbitrary integers

On 10/13/2014 10:29 PM, Daniel Kho wrote:
> Hi Martin,
> Thanks for this. One thing that has been bothering me quite often 
> these days is that integers, booleans, bits (and possibly other types 
> from the std.standard package) do not have the concept of resolution.
>
> While I support the idea of having a completely-unconstrained 
> universal_integer type, I still would like to have the ability to 
> assign an "invalid" or "undriven" value to an integer. Like this:
>
> signal i: integer;
> ...
> begin
>     i <= NaN;
>
> where NaN is an invalid value for an integer. I find myself needing 
> the ability to assign integers to an invalid / undriven state, during 
> initialisation/reset for example, or when I would want to have a 
> driver explicitly "release" the bus, while have another driver drive it.
>
A "NaN" is a very specific value for a floating point number.   An 
exponent of all "1" (which means infinity) and a fraction which starts 
with a "1".     Since all of the bits in an integer are valid, I don't 
know how you would do an invalid number.   You can't just pick one 
because somebody will need to use it.
> In the past, I tried writing resolution functions for integers to do 
> exactly this, but found my solution inadequate as I need to reserve a 
> valid integer, e.g. x"ffff_ffff" to act as my invalid/undriven 
> integer. I find that when writing such resolution functions, I do need 
> checks against an undriven state. For std_logic, we already have the 
> function "is_x" (and I also have my own "is_01") which we could use 
> when resolving std_ulogic type signals. But for integer and boolean 
> types, I can't think of a way to do the same thing as an "is_x" 
> function would for std_logic. My dirty hack was just to reserve a very 
> large integer to act as my invalid value.
>
> My opinion is that it would be great if we can do similar things with 
> integers, booleans, and bits. It is often easier to design using these 
> datatypes than with std_logic(_vector).
>
> What do you think?
>
> Best regards,
> Daniel
>
> On 10 October 2014 19:07, Martin.J Thompson <Martin.J.Thompson@trw.com 
> <mailto:Martin.J.Thompson@trw.com>> wrote:
>
>     Hi all,
>
>
>     After the discussions in the last telecon, I have updated the
>     Arbitrary Integers page:
>
>     http://www.eda-twiki.org/cgi-bin/view.cgi/P1076/ArbitraryIntegers
>
>     In summary, the proposal is to
>
>     ·Expose universal_integer as a completely unconstrained integer
>     type, (and possibly make INTEGER a constrained subtype of this. 
>     Each tool will constrain the INTEGER to the range it currently
>     provides for backwards compatibility.)
>
>     ·Create universal_integer’high and ‘low
>
>     ·Allow logical and shift operations on universal_integers
>
>     ·Question: should conversions between UNIVERSAL_INTEGER and
>     INTEGER be implicit or explicit?
>
>     Any comments you have will be gratefully received as always :)
>
>     Thanks,
>
>     Martin
>
>
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