Re: How do I write directly to a memory address?

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 02/03/2011 01:08 AM, mockturtle wrote:
>
> I do not have any experience about this part of Ada, but maybe I can give you
> a "pointer" to some bootstrapping information.  Maybe you could want to use
> an attribute representation  clause for Vidmem'Address (see Section 13.3 of
> the RM) . With some improvisation, I would write something like
>
> type Vidmem_Array is array (natural range<>) of Character; Vidmem :
> Vidmem_Array (0 .. Max_Size);
>
> for Vidmem'Address use 16#000B_8000#;

An address clause is the usual approach. If one is going to write to such 
memory, one probably doesn't care what the Ada runtime might do to initialize 
it, but if one is planning to read predefined information stored there, one 
probably wants to prevent any initialization through a pragma Import:

pragma Import (Ada, Vidmem);

System.Address is an implementation-defined type, and an address clause takes a 
value of that type. A universal integer may or may not be acceptable. Any such 
code is not portable.

Other possibilities include Ada.Storage_IO (ARM A.9) and 
System.Address_To_Access_Conversions (ARM 13.7.2). Masochists (such as those who 
use C-family languages by choice) might want to use address arithmetic, found in 
System.Storage_Elements (ARM 13.7.1).

Note that the quoted C code will happily read from memory not part of message 
and write to memory not part of vidmem; Ada will not.

-- 
Jeff Carter
"How'd you like to hide the egg and gurgitate
a few saucers of mocha java?"
Never Give a Sucker an Even Break
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