.section .data
 .global op1
 .global op2
 .global res

.section .text
 .global sum # void sum(void)
 sum:

# prologue
pushl %ebp # save previous stack frame pointer
movl %esp, %ebp # the stack frame pointer for sum function
movl op1, %ebx #place op1 in ebx
movl op2, %eax #place op2 in eax
addl %ebx, %eax #add ebx to eax. Result is in eax
movl %eax, res # copy the result to res

# epilogue
movl %ebp, %esp # restore the previous stack pointer ("clear" the stack)
popl %ebp # restore the previous stack frame pointer
ret