Read from stdin and print input - AT&T assembly

read.s

#################
# How to run	#
#################
# $ as --gstabs read.s -o read.o
# $ ld read.o -o read
# $ ./read
################

.text

.globl _start

MAX_CHAR=30

_start:

	## Start message ##
	movl $4, %eax
	movl $1, %ebx
	movl $msg, %ecx
	movl $len, %edx
	int $0x80


	## READ ##
	movl $3, %eax 		#sys_read (number 3)
	movl $0, %ebx		#stdin (number 0)
	movl %esp, %ecx 	#starting point
	movl $MAX_CHAR, %edx	#max input
	int $0x80		#call


	## Need the cycle to count input length ##	
	movl $1, %ecx 		#counter
end_input:
	xor %ebx, %ebx
	movb (%esp), %bl
	add $1, %esp		#get next char to compare 
	add $1, %ecx	 	#counter+=1
	cmp $0xa, %ebx		#compare with "\n" 
	jne end_input		#if not, continue 


	## WRITE ##
	sub %ecx, %esp		#start from the first input char
	movl $4, %eax		#sys_write (number 4)
	movl $1, %ebx		#stdout (number 1)
	movl %ecx, %edx		#start pointer
	movl %esp, %ecx		#length
	int $0x80		#call
	 

	## EXIT ##
	movl $1, %eax
	int $0x80	

.data
msg: .ascii "Insert an input:\n"
len =.-msg

xor usding for what ?

xor usding for what ?

https://stackoverflow.com/questions/1396527/what-is-the-purpose-of-xoring-a-register-with-itself