Re: Fortran's Equivalence

[okellogg@cube.net (Oliver Kellogg)]

Michael F Brenner (mfb@mbunix.mitre.org) wrote:
> [...]                         The portability problem comes in because
> the Ada-95 manual did not standardize a bit numbering in the word, nor
> does it standardize a pragma to tell which bit is zero in a portable
> manner. 

Portability is nevertheless not impossible, just more difficult to
achieve. For porting code between a host computer and a 1750A target, we
wrote following package:

-- package Bitranges, 1750A version

with System;

package Bitranges is    -- MIL-STD-1750A version

  Unit : constant := System.Storage_Unit;

  subtype Bit_00_00 is Natural range 15..15;
  subtype Bit_00_01 is Natural range 14..15;
  subtype Bit_00_02 is Natural range 13..15;
  subtype Bit_00_03 is Natural range 12..15;
  subtype Bit_00_04 is Natural range 11..15;
  subtype Bit_00_05 is Natural range 10..15;
  subtype Bit_00_06 is Natural range 09..15;
  subtype Bit_00_07 is Natural range 08..15;
  subtype Bit_00_08 is Natural range 07..15;
  subtype Bit_00_09 is Natural range 06..15;
  subtype Bit_00_10 is Natural range 05..15;
  subtype Bit_00_11 is Natural range 04..15;
  subtype Bit_00_12 is Natural range 03..15;
  subtype Bit_00_13 is Natural range 02..15;
  subtype Bit_00_14 is Natural range 01..15;
  subtype Bit_00_15 is Natural range 00..15;

  subtype Bit_01_01 is Natural range 14..14;
  subtype Bit_01_02 is Natural range 13..14;
  subtype Bit_01_03 is Natural range 12..14;
  subtype Bit_01_04 is Natural range 11..14;
  [...]
  subtype Bit_01_14 is Natural range 01..14;
  subtype Bit_01_15 is Natural range 00..14;

  subtype Bit_02_02 is Natural range 13..13;
  subtype Bit_02_03 is Natural range 12..13;
  [...]
  subtype Bit_02_15 is Natural range 00..13;

  [...]

  subtype Bit_14_14 is Natural range 01..01;
  subtype Bit_14_15 is Natural range 00..01;

  subtype Bit_15_15 is Natural range 00..00;

end Bitranges;


-- Package bitranges spec, host computer version

with System;

package Bitranges is    -- host computer native version

  Unit : constant := System.Storage_Unit;

  subtype Bit_00_00 is Natural range 00..00;
  subtype Bit_00_01 is Natural range 00..01;
  subtype Bit_00_02 is Natural range 00..02;

  [...]

  subtype Bit_15_15 is Natural range 15..15;

end Bitranges;


-- Representation clauses must then be written e.g. as follows:

with Bitranges;  use Bitranges;

package Example is

  type My_Rec is record
    A : Short_Integer;
    B : Short_Integer;
  end My_Rec;

  for My_Rec use record
    A  at 0*Unit range Bit_00_01'first..Bit_00_01'last;
    B  at 0*Unit range Bit_02_15'first..Bit_02_15'last;
  end record;

end Example;