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

You don’t need to reserve an unused number, just make the bus resolve to that number if everyone drives it.

For an integer resolution function, you can use 0 as the undriven value, and the resolution function just sums the drivers. If more than one driver is on (non-zero), have the resolution function assert a warning, and report the non-zero values if you like. The only thing that won’t catch is if all but one driver are intending to drive 0, and one mistakenly does not.

Andy

From: owner-vhdl-200x@eda.org [mailto:owner-vhdl-200x@eda.org] On Behalf Of Daniel Kho
Sent: Monday, October 13, 2014 9:29 PM
To: vhdl-200x@eda.org
Subject: EXTERNAL: Re: [vhdl-200x] Update to proposal for arbitrary integers

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.
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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