Re: EXTERNAL: Re: [vhdl-200x] Modular types

On 8 Jul 2014, at 8:24 pm, tgingold@free.fr wrote:

> Well, the arithmetic is not the same.  Eg: (3 + 1) mod 3
> 
> For a signed type, the result is 1.
> For a modular type with a modulus of 4, the result is 0.

And they aren't meant to return the same value.

3.5.4 Integer Types
http://www.adaic.org/resources/add_content/standards/05rm/html/RM-3-5-4.html

For every modular subtype S, the following attributes are defined: 

   S'Mod    S'Mod denotes a function with the following specification:

                function S'Mod (Arg : universal_integer)
                   return S'Base
                 This function returns Arg mod S'Modulus, as a value of 
                 the type of S.

S'Modulus       S'Modulus yields the modulus of the type of S, as a 
                value of the type universal_integer. 

The 'Mod attribute is a function that performs modulus reduction for the type of S.

Ada's modular integers use 0 as 'LOW bound  (also from 3.5.4):

"...The set of values for a modular integer type are the values from 0 to one less than the modulus, inclusive."


3 is a valid value of the modular type, as is 0:

    type mod4 is mod 4;

That two bit counter counts "0, 1, 2, 3, 0,...".

3 isn't a valid value for a modular type defined mod 3.

If you wanted the same answer you could have used the same modulus.  





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