Gcc Inline ASM input variable

Question

I've this code:

void  geninterrupt (int x) {
    __asm__(
    "movb x, %al \n"
        "movb %al, genint+1 \n"
        "jmp genint \n"
    "genint: \n"
        "int $0 \n"
    );
}

How can I make movb use the argument of geninterrupt()?

See ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss5.2. — dbrank0
– dbrank0, Commented Oct 14, 2013 at 20:42

Carl Norum · Accepted Answer · 2013-10-14 20:49:41Z

2

You need to use the constraints fields correctly:

void  geninterrupt (int x) {
  __asm__("  movb %[x], %%al \n"
          "  movb %%al, genint+1 \n"
          "  jmp genint \n"
          "genint: \n"
          "  int $0 \n"
         : /* no outputs */
         : [x] "m" (x) /* use x as input */
         : "al" /* clobbers %al */
         );
}

Here's a good how-to about GCC inline assembly and a link to the relevant GCC documentation.

Edit: since your GCC seems to not be able to handle the labeled operands

edited Oct 14, 2013 at 20:49

answered Oct 14, 2013 at 20:44

Carl Norum

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3 Comments

Luca D'Amico Over a year ago

Thanks Carl (for both this and the pack(pop) question, I'm having a lot of difficulties tonight..), but I got this error with your code: invalid 'asm': operand number missing after %-letter __asm__(" movb %[x], %al \n"

Carl Norum Over a year ago

Whoops - missed some % signs. Fixed now. I also had to change to an m constraint.

Luca D'Amico Over a year ago

IT WORKS!!!! Thank you very much mate!! Also thank you very much for both the useful links !! :)

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