# introductory example; finds the sum of the elements of an array

# assembly/linking Instructions:

# as -a --gstabs -o add4.o Add4.s
# ld -o add4 add4.o

.data  # start of data segment

x:
      .long   1
      .long   5
      .long   2
      .long   18

sum:
      .long 0

.text  # start of code segment

.globl _start
_start:
      movl $4, %eax  # EAX will serve as a counter for
                     # the number words left to be summed
      movl $0, %ebx  # EBX will store the sum
      movl $0, %ecx  # ECX will serve as an offset, relative to x, of the
                     # current element to be summed
top:  addl x(%ecx), %ebx  # see comment at end below
      addl $4, %ecx  # move pointer to next element
      decl %eax  # decrement counter
      jnz top  # if counter not 0, then loop again
done: movl %ebx, sum  # done, store result in "sum"

# The add instruction in the line labeled "top" uses "indexed" addressing
# mode, in which an operand is specified as being a register-number of
# bytes past a constant base.  Here, x(%ecx) means "ECX bytes past x."  So
# for example to look at the third element of x, ECX must contain 8.
#
# Any constant can be used as the base.  For example, (x+4)(%ecx) would
# refer to the word ECX bytes past x+4 (or, if you prefer to view it
# this way, 4+ECX bytes past x).