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

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Thu, 03 Feb 2011 09:08:50 +0100, mockturtle <framefritti@gmail.com> a  
écrit:

>> Below is an
>> example of code I would like to be able to implement.
>>
>> ...
>> unsigned int print(char *message, unsigned int line)
>> {
>> 	char *vidmem = (char *) 0xb8000;
>> 	unsigned int i= 0;
>>
>
> 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#;

There may be a real issue here. This was segment register = B800 and  
offset register = 0000, for the base address. Ada does not make any  
distinction about segment and offset. There should be something stated in  
the compiler's documentation, to know how it maps linear address to  
segment:offset address. Otherwise, I feel the result is unpredictable  
because ambiguous: the compiler may have at least two options. The first  
one would be to work with a fixed segment as the base address for each  
memory pool and then the address attribute would match the offset only  
(Pascal compilers was working this way inside of units). The second one  
would be to use the standard convention at that time, to have offset range  
in 0..15 and the remaining of the address held by the segment… the was the  
Borland C++ way when using the so called Huge Memory model. These two  
interpretation ends into different things. In short, there is no way to  
believe “for Vidmem'Address use 16#000B_8000#” can be OK without further  
investigations.

-- 
Si les chats miaulent et font autant de vocalises bizarres, c’est pas pour  
les chiens.

“I am fluent in ASCII” [Warren 2010]