Thread: how to use variables within __asm__()?

  1. #1
    template<typename T> threahdead's Avatar
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    how to use variables within __asm__()?

    hi!

    how do i use C variables within a __asm__() statement?

    assume i have the following function:
    Code:
    void doit(void)
    {
      __asm__("movl $0x1, %eax\n"
                    "movl $0x1, %ebx\n"
                    "int $0x80\n");
    }
    and i want to use hardcoded variables instead of $0x1 for example.

    how would i do that?
    i tried
    Code:
    __asm__(".include \"defines.asm\"\n");
    but that didnt seem to work.

    can i hardcode the values of the variables in a C header file and use them afterwards from __asm__()?

    thanks

  2. #2
    and the hat of int overfl Salem's Avatar
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    That would depend entirely on which C compiler you have.
    Everything about asm() is implementation-specific.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    template<typename T> threahdead's Avatar
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    im using gcc 3.3.1 on a linux x86 machine...

  4. #4
    and the hat of int overfl Salem's Avatar
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    Try here
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
    Visionary Philosopher Sayeh's Avatar
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    No matter which implementation you use, you have to understand that an assembler may only deal with variables via registers or register references to RAM.

    Registers are limited in number. The more 'C' variables you use in an inline assembler function, the more the compiler has to juggle real registers to handle your needs.
    It is not the spoon that bends, it is you who bends around the spoon.

  6. #6
    template<typename T> threahdead's Avatar
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    thanks for the replies. i think i got it working now.

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