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Re: x64 address indexed by 32-bit address

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On 19/01/2024 14:42, Paul Edwards wrote:
> Hi.
> 
> I am using a slightly modified GCC 3.2.3
> to generate x64 code, and using PDPCLIB
> and running under Linux x64.
...
> Now I have this code:
...
> printf("as negative is %x\n", p[-1]);

> which is generating (for that last line):
> 
> LM1873:
>          movl    $4294967295, %eax
>          addq    -64(%rbp), %rax
>          movsbl  (%rax),%esi
>          movl    $LC445, %edi
>          movb    $0, %al
>          call    _printf

> That first instruction - the movl - has
> negative 1 as an unsigned value. 

of course because it's just the lower part of RAX
where the upper part become zeroed by a move to EAX.

> I tried manually changing the generated assembler
> to $-1 but the result appears to be the
> same (I may have stuffed up the test).

you have to make RAX -1
XOR RAX,RAX
DEC RAX
but what's wrong with: DEC instead of ADD -1
__
wolfgang

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Re: x64 address indexed by 32-bit address wolfgang kern <nowhere@never.at> - 2024-01-30 10:14 +0100
  Re: x64 address indexed by 32-bit address wolfgang kern <nowhere@never.at> - 2024-02-01 09:29 +0100
  Re: x64 address indexed by 32-bit address Paul Edwards <mutazilah@gmail.com> - 2024-02-02 16:12 +0800
    Re: x64 address indexed by 32-bit address wolfgang kern <nowhere@never.at> - 2024-02-02 14:16 +0100
  Re: x64 address indexed by 32-bit address Paul Edwards <mutazilah@gmail.com> - 2024-02-13 14:24 +0800

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