Re: [vhdl-200x-ft] More ideas on 1164 and 1076.3

Ryan,
If I want to mix signed and unsigned, I would rather
make it happen by type conversion. I think it better
illuminates the code and preserves the VHDL intent of
type safety.

   signal A_uv : unsigned(6 downto 0) ;
   signal B_sv : signed(7 downto 0) ;
   signal Y_sv : signed(7 downto 0) ;

   Y_sv <= B_sv + signed('0' & A_uv) ;

Back to literals,
without these additional functions the bit string literals
that are used with a given type are of that particular
type. Consider multiplication:

   signal A_sv : signed(7 downto 0) ;
   signal Y_sv : signed(11 downto 0) ;

   Y_sv <= A_sv * "0110" ;

Include these functions and you must add a type qualifier:
   Y_sv <= A_sv * signed'("0110") ;

The sizing rule for integers assumes that the integer
argument is the same size of as the array argument.
Hence, for the following to be legal, Y_sv must be
16 bits rather than 12:

   signal A_sv : signed(7 downto 0) ;
   signal Y_too_big_sv : signed(15 downto 0) ;

   Y_too_big_sv <= A_sv * 6 ;

Cheers,
Jim

> Jim,
>
> My personal opinion is the opposite of yours in this case.
>
>
>>>Another one is that in the numeric_std package, often you need
>>>to do operations on both signed and unsigned numbers. The way
>>>we handle this now with casting is clumsy.
>>>
>>><proposal>
>>>Add the following functions to numeric_std and numeric_bit:
>>> function "+"(L : UNSIGNED; R: SIGNED) return SIGNED;
>>> function "+"(L : SIGNED; R: UNSIGNED) return SIGNED;
>>
>>. . .
>>
>>> function "<="(L : UNSIGNED; R: SIGNED) return boolean;
>>> function "<="(L : SIGNED; R: UNSIGNED) return boolean;
>>></proposal>
>>
>>This would be bad. Currently this is done in std_logic_arith
>>and causes ambiguity with literals:
>>
>> signal A_sv, Y_sv : signed(7 downto 0) ;
>>
>> Y_sv <= A_sv + "1111" ; -- is it signed or unsigned.
>
>
> I have written my own versions of these functions, and I use them often.
>
> Is the literal signed or unsigned? Excellent question! Should it be signed
> just because the other operand is signed? I don't see an unambiguous
> "right" way to interpret the bit literal, so I think we're missing useful
> functionality to preserve an ambiguous interpretation of intent. (I think
> using a qualified expression or integer is a better indication of intent.)
>
> ---
> Ryan Hinton
> L-3 Communications / Communication Systems - West
> ryan.w.hinton@L-3com.com
>
>

-- 
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Jim Lewis
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