#	;; cmp.s

#	should be a replacement for /bin/cmp ... ;-)
#

#include <asm/unistd_32.h>
# #include <asm-generic/fcntl.h>	# I'd like to use this, but... it's not feasible as far as I can tell


	.file	"cmp.s"

	.data

_argc:				# argument count
	.long 	0
_argv:				# pointer to arguments
	.long 	0
_exit_status:			# exit status, initially zero, change it if and when we have an error
	.long	0

_byte_counter:			# byte counter
	.long 	1
_line_counter:			# line counter
	.long 	1
_fd1:				# file descriptor 1
	.long	0
_fd2:				# file descriptor 2
	.long 	0

	
_buffer0:
	.space	4096,0
_buffer1:
	.space	4096,0

	.text
	.globl 	cmp

cmp:
#	xor	%r11,%r11	# exit status, change it if and when we have an error
#	mov	$1,%r12		# we'll use %r12 for our byte counter
#	xor	$1,%r13		# we'll use %r13 for our line counter

_break:	
	pop	%eax		# this is argc...
	cmp	$3,%eax		# check that we have the right number of arguments
	jne	_error_finish

	pop 	%ebx		# remove command
	dec	%eax
	jz	_finish		# check for no args left in argv

	# open first file
	mov	$0,%ecx		# not "xor" since we want to make it clear that this is $0 = O_RDONLY, not just any zero!
	pop	%ebx		# this is the next *argv --- go ahead and put it in %rdi since we are going to try to open it for reading
	mov	$__NR_open,%eax	
	int	$0x80
	cmp	$0,%eax
	jl	_error_finish
	mov	%eax,_fd1	# save file descriptor in _fd1

	# open second file
	mov	$0,%ecx		# not "xor" since we want to make it clear that this is $0 = O_RDONLY, not just any zero!
	pop	%ebx		# this is the next *argv --- go ahead and put it in %rdi since we are going to try to open it for reading
	mov	$__NR_open,%eax	
	int	$0x80
	cmp	$0,%eax
	jl	_error_finish

	mov	%eax,_fd2	# save file descriptor in _fd2

_loop:
	# read one byte from file 0 into buffer0
	mov	_fd1,%ebx	# recover file descriptor from %r14
	lea	_buffer0,%ecx	# buffer location
	mov	$1,%edx		# how many bytes
	mov	$__NR_read,%eax
	int	$0x80
	cmp	$0,%eax
	jl	_error_finish
	je	_file0_finish

	# read one byte from file 1 into buffer1
	mov	_fd2,%ebx	# recover file descriptor from %r15
	lea	_buffer1,%ecx	# buffer location
	mov	$1,%edx		# how many bytes
	mov	$__NR_read,%eax
	int	$0x80
	cmp	$0,%eax
	jl	_error_finish
	je	_file1_finish

	mov	(_buffer0),%eax
	cmp	%eax,(_buffer1)
	jne	_difference_found

	incl	_byte_counter	# byte count

	# check to see if end-of-line
	cmp	$12,%eax 	# if memory serves...
	jne	_loop
	incl	_byte_counter		# line count
	jmp	_loop


_file0_finish:
	# check to see if file2 is finished also --> if so, same
	#                                        --> if not, then different
	mov	_fd2,%ebx
	lea	_buffer1,%ecx
	mov	$1,%edx
	mov	$__NR_read,%eax
	int	$0x80
	cmp	$0,%eax
	je	_finish
	jl	_error_finish
	# or we fall through to "different" finish
	
_file1_finish:
_difference_found:
	mov	$1,%ebx
	mov	$__NR_exit, %eax	# int	$0x80 = 60 is exit
	int	$0x80
	

_error_finish:
	mov	$2,%ebx
	mov	$__NR_exit, %eax	# int	$0x80 = 60 is exit
	int	$0x80


_finish:
	mov	_exit_status,%ebx	# and exit with a status
	mov	$__NR_exit,%eax		# 
	int	$0x80