gistfile1.c

// gcc -Wall -nostdlib -o hello hello.c
void _start() {
    const char msg[] = "Hello World\n";
    const unsigned long len = sizeof(msg);
    asm volatile
    (
	// write(STDOUT_FILENO, msg, len)
        "mov $1, %%rdi\n\t" // stdout is 1
        "mov %0, %%rsi\n\t" // addr of msg
        "mov %1, %%rdx\n\t" // len of msg
        "mov $1, %%rax\n\t" // write is syscall 1
        "syscall\n\t"

	// exit(0)
        "xor %%rdi, %%rdi\n\t" // zero rdi
        "mov $60, %%rax\n\t"   // exit is syscall 60
        "syscall"
        :
        : "g"(msg), "g"(len)
        : "%rdi", "%rsi", "%rdx", "%rcx", "%r11"
    );
}