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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Question about ordinary fixed point types.
Date: Mon, 16 Aug 2010 10:23:56 +0100
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Date: 2010-08-16T10:23:56+01:00
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"Peter C. Chapin" <chapinp@acm.org> writes:

> I need to do real number computations on a microcontroller without a
> floating point unit. I believe the various numeric quantities that I
> need can be normalized into fairly narrow ranges so it seemed to me that
> this is a perfect application of fixed point types. The compiler stores
> the numbers as integers representing the appropriate multiple of Small.
>
> I'm not using GNAT on the microcontroller but if the Ada compiler I am
> using does the same thing as GNAT it will be able to fit Angle_Type as
> it is currently defined into a 16 bit word and compute with it using
> integer instructions. In my environment that is essential.

It would be very normal in that sort of environment to work in binary
fractions of 180 degrees.

with Ada.Numerics;
with Ada.Text_IO; use Ada.Text_IO;
procedure Angles is
   Angle_Type_Delta : constant := Ada.Numerics.Pi * 2.0 ** (-16);
   type Angle_Type is
     delta Angle_Type_Delta
       range -Ada.Numerics.Pi / 2 .. Ada.Numerics.Pi / 2;
   for Angle_Type'Small use Angle_Type_Delta;
   for Angle_Type'Size use 16;
   Count_Of_Values : Positive;
   Value : Angle_Type;
begin
   Put_Line ("angle_type'first: "
               & Long_Float'Image (Long_Float (Angle_Type'First)));
   Put_Line ("angle_type'last: "
               & Long_Float'Image (Long_Float (Angle_Type'Last)));
   Value := Angle_Type'First;
   Count_Of_Values := 1;
   loop
      exit when Value = Angle_Type'Last;
      Count_Of_Values := Count_Of_Values + 1;
      Value := Value + Angle_Type'Small;
   end loop;
   Put_Line ("count_of_values: " & Positive'Image (Count_Of_Values));
end Angles;

$ ./angles 
angle_type'first: -1.57079632679490E+00
angle_type'last:  1.57074838989528E+00
count_of_values:  65536