Mips swapping without temp variable

[jiggleswiggly]

so i have swap.s

	.text
main:
	la	$a0,n1
	la	$a1,n2
	jal	swap
	li	$v0,1	# print n1 and n2; should be 27 and 14
	lw	$a0,n1
	syscall
	li	$v0,11
	li	$a0,' '
	syscall
	li	$v0,1
	lw	$a0,n2
	syscall
	li	$v0,11
	li	$a0,'\n'
	syscall
	li	$v0,10	# exit
	syscall

swap:
	
	

L1: 

	.data
n1:	.word	14
n2:	.word	27

my goal is to:
Modify swap.s to translate the following procedure directly to MIPS
assembly language. The temp variable, like all local variables in C
(when not optimized), is stored on the stack. In other words you
cannot use $t0 to hold temp, though you may need it briefly. Hint: you
will need to use 6 lw/sw instructions.
This exercise is slightly contrived, and could be easier if we let you
optimize and use $t0 to hold the temp variable, part of the point of this
exercise is to see what kind of difference optimization can make.

void swap (int *px, int *py) {
int temp;
temp = *px;
*px = *py;
*py = temp;
}

i am confused on jal and jr's though...
for swap could I just do:

xor $a0 $a0 $a1
	xor $a1 $a0 $a1
	xor $a0 $a0 $a1
	jr $ra

or do i need to save to the stack and such?

 

[chiro]

Hey jiggleswiggly and welcome to the forums.

That piece of XOR code is actually a real swap that works without needing a temporary variable.

If you wanted to prove it then use the properties of XOR and note that A XOR A = 0 and 0 XOR B = B for any B and A.