Re: [vhdl-200x-ft] fixed point package action items and telecon

Proposals to be discusses (part 2):

3) Issue:
Bit sizing rules for multiplication and division
For a multiply in numeric_std:
    signed (a downto 0) * signed (c downto 0) = signed (a+c+1 downto 0)
    unsigned (a downto 0) * unsigned (c downto 0) = unsigned (a+c+1 downto 0)
Currently in the fixed point packages
    ufixed(a downto b) * ufixed(c downto d) = ufixed(a+c downto b+d)
    sfixed(a downto b) * sfixed(c downto d) = sfixed(a+c downto b+d)
Proposed is:
    ufixed(a downto b) * ufixed(c downto d) = ufixed(a+c+1 downto b+d)
    ufixed(a downto b) * sfixed(c downto d) = sfixed(a+c+1 downto b+d)
    sfixed(a downto b) * sfixed(c downto d) = sfixed(a+c downto b+d)

For division in numeric_std:
    signed (a downto 0) / signed (c downto 0) := signed (a downto 0);
    unsigned (a downto 0) / unsigned (c downto 0) := unsigned (a downto 0);
Currently in the fixed point package
    ufixed (a downto b) / ufixed (c downto d) := ufixed (max(a,c) downto min(b,d))
    sfixed (a downto b) / sfixed (c downto d) := sfixed (max(a,c) downto min(b,d))
Proposed:
     ufixed(a downto b) / ufixed(c downto d) = ufixed(a-d+1 downto b-c-1)
     ufixed(a downto b) / sfixed(c downto d) = sfixed(a-d+1 downto b-c)
     sfixed(a downto b) / ufixed(c downto d) = sfixed(a-d downto b-c-1)
     sfixed(a downto b) / sfixed(c downto d) = sfixed(a-d downto b-c)
Note that from several post (and the problem I am currently having with
the packages) they appear to be problematic.

Alternatives:
1) do nothing (once again you can use a "resize")
2) Follow these precision

Pro:
These maintain precision, and actually follow the "numeric_std" idea
for multiply more closely.
Pro:
Division in fixed point is very different than Division in "numeric_std".
Con:
Makes it more difficult to understand division.

Recommendation:
Accept the proposal

4) Issue:
Overloading.
Initially, we have
ufixed <= ufixed + ufixed, sfixed <= sfixed + sfixed;
We automatically add the following:
ufixed <= ufixed + integer, ufixed <= ufixed + real; (same for sfixed).
Next, if we add (and these already exist in the package:)
ufixed <= ufixed + unsigned, sfixed <= sfixed + signed.
At this point we can no longer add bit literals. The reasons are two:
1) The bit literal is not bounded (our type is ufixed is array (integer range
<>) of std_logic;)
2) Since we already have "ufixed" and "unsigned" there is a type conflict.
Thus ALL bit liters must be either casted or assigned to a type
before being used.

The proposal is to add the following functions:
b) sfixed := ufixed - ufixed;
c) sfixed := sfixed + ufixed; (- * /)
d) sfixed := ufixed + signed; (- * /) (once a signed number is involved
the result would be "sfixed")

Pros:
People need to do this anyway
Pro:
You already MUST cast any bit literals
Con:
more functions to maintain and test
Con:
Doesn't have the same restrictions as "numeric_std".

Work around:
sfixed := sfixed + to_sfixed (ufixed);

Recommendation:
Accept the proposal.

-- 
David W. Bishop dbishop@vhdl.org       All standard disclaimers apply.