Question

is it possible to write a single character using a syscall from within an inline assembly block? if so, how? it should look “something” like this:

__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " movl $80, %%ecx \n\t"
                     " movl $0,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );

$80 is ‘P’ in ascii, but that returns nothing.

any suggestions much appreciated!

Answer 1

Something like

char p = 'P';

int main()
{
__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " leal p , %%ecx \n\t"
                     " movl $0,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );
}

Add: note that I’ve used lea to Load the Effective Address of the char into ecx register; for the value of ebx I tried $0 and $1 and it seems to work anyway …

Avoid the use of external char

int main()
{
__asm__ __volatile__
                    (
                     " movl $1,  %%edx \n\t"
                     " subl $4, %%esp \n\t"
                     " movl $80, (%%esp)\n\t"
                     " movl %%esp, %%ecx \n\t"
                     " movl $1,  %%ebx \n\t"
                     " movl $4,  %%eax \n\t"
                     " int $0x80       \n\t"
                     " addl $4, %%esp\n\t"
                     ::: "%eax", "%ebx", "%ecx", "%edx"
                    );
}

N.B.: it works because of the endianness of intel processors! 😀