Re: Simulation of fixed point in c WITH DIFFERENT BIT-WIDTH

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Simon Wright <simon@pushface.org> writes:

> I'm not sure what is actually required for portability here. AARM95
> 3.5.9(22-24) says
> 
>    The base range of an ordinary fixed point type need not include the
>    specified bounds themselves so that the range specification can be
>    given in a natural way, such as:
> 
>    type Fraction is delta 2.0**(-15) range -1.0 .. 1.0;
> 
>    With 2's complement hardware, such a type could have a signed
>    16-bit representation, using 1 bit for the sign and 15 bits for
>    fraction, resulting in a base range of -1.0 .. 1.0-2.0**(-15).

This is a language design flaw, in my opinion.  "T'Last in T" and
"T'Last in T'Base" should both always be true.  If you want 16 bits, you
should say "-1.0 .. 1.0-2.0**(-15)."  After all, we don't play these
games with integers -- if you say "range -2**15..2**15", you get at
least 17 bits (probably 32 bits).

The "need not" in the above wording means that the implementation has a
choice.  It can include 1.0 in the base range of Fraction, or not.
Some implementations will include the upper bound if and only if it
does not require extra bits.  I think that kind of non-portability
is unhelpful to programmers.

- Bob