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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Thu, 3 Feb 2011 00:24:38 -0800 (PST)
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Date: 2011-02-03T00:24:38-08:00
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Syntax Issues wrote on comp.lang.ada:
> I am planning to start learning lower-level programming with ada,
> however, I was unable to find a solid tutorial on writing directly to
> a memory address or interfacing with assembly. Does anyone know where
> I can find a reference to some tutorials/information? Below is an
> example of code I would like to be able to implement.
>
> ...
> unsigned int print(char *message, unsigned int line)
> {
> =A0 =A0 =A0 =A0 char *vidmem =3D (char *) 0xb8000;
> =A0 =A0 =A0 =A0 unsigned int i=3D 0;
>
> =A0 =A0 =A0 =A0 i=3D(line*80*2);
>
> =A0 =A0 =A0 =A0 while(*message!=3D0) // 24h
> =A0 =A0 =A0 =A0 {
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 vidmem[i]=3D *message;
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 *message++;

*Every* time I look at C code, I see a bug.
I think this is a bug; it should read "message++" since you want to
increment the pointer, not the pointed-to character.

> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 i++;
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 vidmem[i]=3D 0x7;
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 i++;
> =A0 =A0 =A0 =A0 };
>
> =A0 =A0 =A0 =A0 return(1);};
>
> ...

The proper way to achieve what you want is not to translate the
(buggy) C code into Ada; instead, use Ada to model the problem and
provide a clean abstraction.  In this case, the model is that the
video memory, located at some address, is a two-dimensional array of
cells; each cell contains a character and an attribute byte.

type Cell_T is record
   Char : Character;
   Attribute : Interfaces.Unsigned_8;
end record;
pragma Pack (Cell_T);

type Horiz_Coordinate_T is mod 80;
type Vert_Coordinate_T is mod 25;
type Console_T is array
  (Horiz_Coordinate_T, Vert_Coordinate_T) of Cell_T;
pragma Pack (Console_T);

Console : Console_T;
for Console'Address use 16#b8000#; -- the only low-level trick!

procedure Put (C            : in Character;
               Into_Console : in out Console_T;
               At_X         : in     Horiz_Coordinate_T;
               At_Y         : in     Vert_Coordinate_T) is
begin
   Into_Console (At_X, At_Y) :=3D
     (Char =3D> C, Attribute =3D> 16#7#);
end Put;


procedure Put (S            : in     String;
               Into_Console : in out Console_T;
               At_X         : in     Horiz_Coordinate_T;
               At_Y         : in     Vert_Coordinate_T) is
  X : Horiz_Coordinate_T :=3D At_X;
  Y : Vert_Coordinate_T :=3D At_Y;
begin
  for J in S'Range loop
    Put (C =3D> S (J),
         Into_Console =3D> Into_Console,
         At_X =3D> X,
         At_Y =3D> Y);
    X :=3D X + 1; -- modular arithmetic: wraps around
    if X =3D 0 then
       Y :=3D Y + 1; -- ditto
    end if;
  end loop;
end Put;

Hope this helps

--
Ludovic Brenta.