.lcomm  var1 8

.global _start
_start:

   # load the address of "var1" into register %eax
   movl $2,%eax  # eax = 2
   movl $var1,%ebx
   movb $0xcd,1(%ebx,%eax,2) # ((char*)var1)[eax] = 0xcd
   # ebx is the base register, and it has the address of var1
   # eax is the index register, and it is initialized to 2
   # the scaling factor is 2
   # the displacement is 1
   # the effective address is:
   # %ebx(base reg) + 1(displacement) + (%eax (index reg) * 2 (scaling factor))
   # address of var1 + 1 + (2 * 2)
   # address of var1 + 5
   # the sixth byte from var1 will be initialized to 0xcd
   
   movl $0, %ebx
   movl $1, %eax
   int  $0x80