Re: Memory Access Question

[brians@hpcljms.HP.COM (Brian Sullivan)]

  It is possible to use UNCHECKED_CONVERSION to provide access
memory locations.  The following Ada package will allows you
to read and write to memory locations.

  The file depends on the existence of a predefined integer type
that is the same size as SYSTEM.ADDRESS, and the SYSTEM.ADDRESS
is of the same size as a pointer or access object.

------------------------------------------------------------------------
with SYSTEM;
package MEMORY is

   --
   -- The predefined integer type used for the type ADDR_INT
   -- must be the same size in bits as the type SYSTEM.ADDRESS
   -- because unchecked conversion is used to convert ADDR_INT
   -- into SYSTEM.ADDRESS
   --
   -- Also the size of an access type must be the same as the
   -- size of SYSTEM.ADDRESS
   --
   -- These two requirements are automatically check in the
   -- elaboration code for the body of this package.
   --

   type ADDR_INT is new INTEGER;      

   type DATUM is range 0 .. (2**SYSTEM.STORAGE_UNIT)-1;

   function  READ ( FROM : ADDR_INT ) return DATUM;
   
   procedure WRITE( TO : ADDR_INT; VALUE : DATUM);
   
   function  CONVERT_TO_ADDR_INT( LOC : SYSTEM.ADDRESS) return ADDR_INT; 
   
private

   pragma INLINE(READ);
   pragma INLINE(WRITE);
   pragma INLINE(CONVERT_TO_ADDR_INT);

   for DATUM'SIZE use SYSTEM.STORAGE_UNIT;
   
end MEMORY;   

with UNCHECKED_CONVERSION;
package body MEMORY is

   type DATUM_PTR is access DATUM;
   
   function CONVERT_TO_DATUM_PTR is 
      new UNCHECKED_CONVERSION( SOURCE => ADDR_INT,
                                TARGET => DATUM_PTR);
   pragma INLINE(CONVERT_TO_DATUM_PTR);

   function LOCAL_CONVERT_TO_ADDR_INT is 
      new UNCHECKED_CONVERSION( SOURCE => SYSTEM.ADDRESS,
                                TARGET => ADDR_INT);
   pragma INLINE(LOCAL_CONVERT_TO_ADDR_INT);

   function  READ ( FROM : ADDR_INT ) return DATUM is
   begin
      return CONVERT_TO_DATUM_PTR(FROM).all;
   end;
   
   procedure WRITE( TO : ADDR_INT; VALUE : DATUM) is
   begin
      CONVERT_TO_DATUM_PTR(TO).all := VALUE;
   end;
   
   function  CONVERT_TO_ADDR_INT( LOC : SYSTEM.ADDRESS) return ADDR_INT is
   begin
      return LOCAL_CONVERT_TO_ADDR_INT(LOC);
   end;
   
begin
   if ADDR_INT'SIZE /= SYSTEM.ADDRESS'SIZE then
      raise PROGRAM_ERROR;
   end if;    
   if DATUM_PTR'SIZE /= SYSTEM.ADDRESS'SIZE then
      raise PROGRAM_ERROR;
   end if;    
end MEMORY;