# AddOne.s, example of interfacing C to assembly language
# 
# paired with TryAddOne.c, which calls addone
#
# assemble by typing
#
#    as --gstabs -o AddOne.o AddOne.s
#

# note that we do not have a .data segment here, and normally would not;
# but we could do so, and if we wanted that .data segment to be visible
# from the calling C program, we would have .globl lines for whatever
# labels we have in that segment

.text

# need .globl to make addone visible to ld
.globl addone

addone:

   # will use EBX, and since the calling module might have a value
   # there, better save the latter on the stack
   push %ebx

   # at this point the old EBX is on the top of the stack, then the
   # return address, then the argument, so the latter is at ESP+8

   movl 8(%esp), %ebx  # would be MOV EBX, [ESP+8] in Intel syntax

   incl (%ebx)  # need the (), since the argument was an address

   # restore value of EBX in the calling module 
   pop %ebx

   ret