Segmentation Fault: 11 when running C program

Question

To try to display graphics using C, I am trying to take advantage of C's "inline assembly" feature. I get no errors during compilation, but when I try to run the program, I get this error:

Segmentation Fault: 11

Here is my code:

int main(){
asm("movb 0xc,%ah");
asm("movb $1,%al");
asm("movw $5,%cx");
asm("movw $5,%dx");
asm("int $0xc");
return 0;
}

Constructive criticism appreciated, insults not. Thanks!

Answer 1

First, it looks like you're trying to use BIOS interrupts to do the graphics, but the graphics interrupt is int 10h (0x10) , not 0xc, so you want to call int $0x10.

Second, you can't call most BIOS interrupts from within 32-bit or 64-bit Linux or Windows programs, so make sure you're compiling this for DOS. Otherwise, calling the invoke interrupt opcode on a BIOS interrupt will crash your program. And if you run a newer version of Windows, you'll probably still have to run your compiled program inside of an emulator like DOSBox for it to work properly.

Finally, GCC inline assembly has a certain format to it:

   __asm__ __volatile__ ( 
         assembler template 
       : output operands                  /* optional */
       : input operands                   /* optional */
       : list of clobbered registers      /* optional */
       );

So for example:

int main()
{
  /* Set video mode: */
  __asm__ __volatile__ (
    "movb $0x0, %%ah \n\
     movb $0x13, %%al \n\
     int $0x10"
    :
    :
    :"ax"
  );

  /* Draw pixel of color 1 at 5,5: */
  __asm__ __volatile__ (
    "movb $0xC,%%ah \n\
     movb $1, %%al \n\
     movw $5, %%cx \n\
     movw $5, %%dx \n\
     int $0x10"
   :
   :
   :"ax","cx","dx"
  );

  /* Reset video mode: */
  __asm__ __volatile__ (
    "movb $0x0, %%ah \n\
     movb $0x03, %%al \n\
     int $0x10"
    :
    :
    :"ax"
  );

  return 0;
}

But the optional fields are only really useful if you're writing functions in assembly language and want to pass in arguments from your C code.

Also, I don't have DJGPP and a DOS installation handy, so I can't test any of this code to make sure it works with the 32-bit protected mode binaries it generates, but hopefully I've hit the nail close enough on the head that you can handle the rest yourself. Good luck!