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From: Tomas Cernaj <tcernaj.WITHOUTTHIS@gmx.de>
Subject: Re: Access to fixed memory location
Newsgroups: comp.lang.ada
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Date: 02 Dec 2007 22:00:16 GMT
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Date: 2007-12-02T23:00:16+01:00
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Thank you for your quick response. You've helped me a lot to sort things 
out.

Actually I think there's two problems at hand:

 1) the read/modify/write access

 2) the indirect access

ad 1): As explained by Richard Kenner, the C construct "y |= 32" is by 
definition equivalent to "y = y | 32", so R/M/W is perfectly OK. However, 
writing "Y (5) := True" IMO means that we only want to set the 
corresponding bit, no matter what the rest of the byte/word etc. looks 
like.
I've tried this using a (volatile) bit field in C, and here again gcc 
generates a R/M/W access. (compiler problem...?)

ad 2): I'm not quite sure if I've understood the concept of aliased types 
you mentioned in (a)... :-|
Anyways, I modified the example in my first post to include "pragma 
Volatile (T_Type);". Here's what gcc generates:

        movzbl  50, %eax		; X (4) := True
        orl     $16, %eax
        movb    %al, 50
        movq    testpkg__y(%rip), %rdx	; Y (5) := True
        movzbl  (%rdx), %eax
        orl     $32, %eax
        movb    %al, (%rdx)

So X is accessed directly while Y is accessed indirectly. That's why I 
thought it had something to do with Y'Address not being constant... (I 
mean that whenever one uses the package where Y is declared one can still 
change the value of Y'Address). 

Am I missing a point?

Thanks,
Tomas