product.s

	# Fabian Frei
	#
	# PURPOSE:  This program calculates the remainder of the product of a
	#           set of integer data items (long values).
	#

	# VARIABLES: The registers have the following uses:
	#
	# %rcx - Holds the index of the data item being examined, changed from %rdx
	# %rbx - current data item
	# %rax - product of current multiplication in loop
	# %rdi - for the exit code of our program
	#
	# The following memory locations are used:
	#
	# data_items - contains the item data.  A 0 at the end data_items is used
	#              to terminate the data. Only positive values are allowed.
	#

	.section .data

data_items:                       # These are the data items
	.quad 2,4,3,7,3,4,0
	#.quad 0					  # for testing of zero length data items

	.section .text

	.globl _start
_start:
	movq $0, %rcx             # move 0 into the index register
	movq data_items(,%rcx,8), %rax # load the first byte of data to %rax for multiplication
	cmpq $0, %rax			  # check %rax if first value is zero
	je early_loop_exit		  # if null we exit early and never enter the loop

start_loop:	                  # start loop
	incq %rcx                 # increment data items counter
	movq data_items(,%rcx,8), %rbx # move next data item into rbx
	cmpq $0, %rbx             # check to see if we've hit the end
	je loop_exit 		      # if yes we go to regular loop exit
	imulq %rbx				  # multiply current item with %rax and save it to %rax
	jmp start_loop            # jump to loop beginning

loop_exit:
	#movq $128, %r8 			  # move 128 into %r8 for division.
	idiv $128				  # divide %rax by 128, stores quotient in %rax, remainder in %rdx
	movq %rdx, %rdi			  # move remainder from %rdx into %rdi as exit code
    movq $60, %rax            # 60 is the exit() syscall
    syscall

early_loop_exit:
	movq $1, %rdi		      # move 1 as exit code into %rdi
	movq $60, %rax			  # 60 is the exit() syscall
	syscall