Question

My Code

const int howmany = 5046;
char buffer[howmany];
    asm("lea     buffer,%esi"); //Get the address of buffer
    asm("mov     howmany,%ebx");         //Set the loop number
    asm("buf_loop:");                      //Lable for beginning of loop
    asm("movb     (%esi),%al");             //Copy buffer[x] to al
    asm("inc     %esi");                   //Increment buffer address
    asm("dec     %ebx");                   //Decrement loop count
    asm("jnz     buf_loop");              //jump to buf_loop if(ebx>0)

My Problem

I am using the gcc compiler. For some reason my buffer/howmany variables are undefined in the eyes of my asm. I’m not sure why. I just want to move the beginning address of my buffer array into the esi register, loop it ‘howmany’ times while copying each element to the al register.

Answer 1

Are you using the inline assembler in gcc? (If not, in what other C++ compiler, exactly?)

If gcc, see the details here, and in particular this example:

    asm ("leal (%1,%1,4), %0"
         : "=r" (five_times_x)
         : "r" (x) 
         );

%0 and %1 are referring to the C-level variables, and they’re listed specifically as the second (for outputs) and third (for inputs) parameters to asm. In your example you have only “inputs” so you’d have an empty second operand (traditionally one uses a comment after that colon, such as /* no output registers */, to indicate that more explicitly).