Makefile

program2: runtime.o program2.o
	gcc $^ -o $@

program2.s

.globl main

.comm var_a, 8
.comm var_b, 8

main:   xorq  %rbx, %rbx
 .L0:   call  runtime_read
 .L1:   movq  %rax, var_a
 .L2:   call  runtime_read
 .L3:   movq  %rax, var_b
 .L4:   jmp   .L18
 .L5:   pushq var_a
 .L6:   movq  var_b, %rcx
        popq  %rax
        cqto
        idivq %rcx
 .L8:   movq  %rdx, var_a
 .L9:   pushq var_a
.L10:   xorq  %rcx, %rcx
        popq  %rax
        cmpq  %rcx, %rax
        setne %bl
.L12:   movq  %rbx, %rax
        orq   %rax, %rax
        jz    .L18
.L13:   pushq var_b
.L14:   movq  var_a, %rcx
        popq  %rax
        cqto
        idivq %rcx
.L16:   movq  %rdx, var_b
.L18:   pushq var_b
.L19:   xorq  %rcx, %rcx
        popq  %rax
        cmpq  %rcx, %rax
        setne %bl
        pushq %rbx
.L21:   pushq var_a
.L22:   xorq  %rcx, %rcx
        popq  %rax
        cmpq  %rcx, %rax
        setne %bl
.L24:   movq  %rbx, %rcx
        andq  %rcx, (%rsp)
.L25:   popq  %rax
        orq   %rax, %rax
        jnz   .L5
.L26:   pushq var_a
.L27:   movq  var_b, %rcx
        addq  %rcx, (%rsp)
.L29:   popq  %rdi
        call  runtime_write
.L30:   xorq  %rdi, %rdi
        call  exit

runtime.c

#include <stdio.h>

long long runtime_read() {
  long long value = 0;
  scanf("%lld", &value);
  return value;
}

void runtime_write(long long value) {
  printf("%lld\n", value);
}

$ make
as   -o program2.o program2.s
gcc runtime.o program2.o -o program2
$ ./program2 
84 30
6  # GCD by Euclid
$