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From: Ted Dennison <dennison@telepath.com>
Subject: Re: Bit Manipulations
Date: 1999/12/21
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Date: 1999-12-21T00:00:00+00:00
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In article
<B9063F55105FD21189FE0000F8103B5B0216503E@emss35m04.owg.fs.lmco.com>,
  comp.lang.ada@ada.eu.org wrote:

> I recently was put onto a project which is written in Ada83. As it has
> been some time since I last used Ada, I can't remember how to do basic
> Bit Maipulations (and can't find it in the books I have).
>
> For instance, I get a byte from a hardware port and want to mast off
> the parity bit ( 16#07F#).
>
> I want to do...
>
> char_data := char_data AND 16#07F#

Way one:
type Byte_Bit_Array is array (1..8) of Boolean;
for Byte_Bit_Array'size use 8;
-- Some compilers might not be happy w/o a pragma pack as well.

Char_Data : Byte_Bit_Array;
Parity_Bit : constant Byte_Bit_Array := (8 => True, others => False);
...
Char_Data := Char_Data and not Parity_Bit;

This allows you to manipulate any of the bits any way you please.

Way two:
type Data_Bit_Integer is 0..127;
for Data_Bit_Integer'size use 7;

type Hardware_Byte is record
   Data       : Data_Bit_Integer;
   Parity_Bit : Boolean;
end record;
for Hardware_Byte use record at mod 8;
   Data       at 0 range 0 .. 6;
   Parity_Bit at 0 range 7 .. 7;
end record;
for Hardware_Byte'size use 8;

Char_Data : Hardware_Byte;
...
Char_Data.Parity_Bit := False;

This allows you to mask off the parity bit, and to use the rest of the
bits as a numeric value.

Ada 95 has a third way, via "modular" integers that support both integer
operations and bitwise boolean ops. Its a shame you can't upgrade.

--
T.E.D.


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