x64 linux only. `gcc helloworld_in_extended_asm.c ; ./a.out`

helloworld_in_extended_asm.c

int main(void){
    const char mes[] = "hello, world\n\0";
    asm volatile(
        "movq %0, %%rsi \n\t movq %1, %%rdx \n\t movq $1, %%rax \n\t movq $1, %%rdi \n\t syscall"
        :
        :"r"(mes),"r"(sizeof(mes))
        :
    );
}