Programming assembly for x86-64 bit machines

Following assembly books which are written for x86-32 bit machines on a 64 bit machine can be a pain in the ass. Thankfully I found this site http://www.x86-64.org/documentation/assembly.html
that really clarified things for me. Thus this is a short example of an assembly program written for an x86-64 bit machines demonstrating how a function can be called. The sample function takes a number a returns the square of that number.

# A program to call a function a compute 
# the square of a number.

.section .data

.section .text

.globl _start

_start:

	push $5			#push first argument	

	call square		# call the square function
	movq $1, %rax    #Call to exit

	int $0x80

# Square Function.
# takes a number and squares it.
# returns the squared number.

.type square, @function
square:
	push %rbp			# save old base pointer
	movq %rsp, %rbp     		# make stack pointer the base pointer
	movq 16(%rbp), %rcx   		# Copy the first argument from the
					# stack which is on position two. 
					# Thus its 2 quad-words which requires 16
	imul %rcx, %rcx   		# Square and store the number
	
	mov %rcx, %rbx			# store the return value
						
	movq %rbp, %rsp			# restore the stack pointer
	pop %rbp
	ret

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