/*******************************************************
*         File:  AddXY_Fns_Ref.m           
*         Date:  Oct. 11, 2021
*       Author:  DSS
*     Computer:  acad
*     Assemble:  asm AddXY_Fns_Ref
*      Execute:  AddXY_Fns_Ref   
*      Purpose:  to demonstrate function calls 
*
*		 This program will declare ints x and y by allocating space for them in 
*	 	 main()'s stack frame. x and y will be read inside a function
		  where each is passed by reference 
*		 They will be stored in the addresses passed in as parameters (in registers)
*		 Their sum will be computed and output
*		 Several registers will be aliased
*
*		 It uses minimal code, optimizing several things
*******************************************************/

define(varMem,16)
define(xOffset,0)
define(yOffset,4)
define(xReg,%r12)
define(yReg,%r13)
define(zReg,%r14)

.global main

main:
	push 	%rbp           		# keep stack aligned
	# Make room for x and y on the stack
	subq	$varMem, %rsp		# must also remain divisible by 16

	# Prompt for X 
	leaq 	promptX(%rip), %rdi  	# load format string to proper register
	call    printf          	# print it
	# Read X   
	xor	%rax,%rax
	movq	%rsp, %rdi
	addq	$xOffset, %rdi
	call	readInt
	# Put value in xReg
	movq	%rsp,xReg		# Stack pointer into xReg
	addq	$xOffset, xReg		# xReg now has address of X
	movq	(xReg),xReg		# Load the value of x into xReg 

	# Prompt for Y 
	leaq 	promptY(%rip), %rdi  	# load format string to proper register
	call    printf          	# print it
	# Read Y   
	xor	%rax,%rax
	movq	%rsp, %rdi
	addq	$yOffset, %rdi
	call	readInt
	# Put value in yReg
	movq	%rsp,yReg		# Stack pointer into xReg
	addq	$yOffset, yReg		# xReg now has address of X
	movq	(yReg),yReg		# Load the value of x into xReg 

	# Sum
	xor	zReg, zReg		# Clear the sum register zReg
	addq	xReg, zReg		# Add x to it
	addq	yReg, zReg		# Add y to it

	# Print
	leaq	frmto(%rip),%rdi	# Output format
	mov	xReg,%rsi
	mov	yReg,%rdx
	mov	zReg,%rcx
	xor	%rax,%rax
	call	printf

done:
	addq	$varMem, %rsp		# Deallocate the variables           
	pop	%rbp
	ret

readInt:
	# Address to read to is in %rdi
	movq	%rdi, %rsi		# Copy address to 2nd parameter
	leaq 	fmt_in(%rip), %rdi  	# load format string
	# Read
	call 	scanf
	ret

.data

promptX:	.asciz	"enter x : "
promptY:	.asciz	"enter y : "
fmt_in:		.asciz	"%d"
frmto:	.asciz	"%d + %d = %d\n"