MIPS swap procedure/function Questions - stack

Question

I'm new to MIPS and while I sort of understand how to do basic tasks with it, I'm struggling heavily with procedures/functions.
I was hoping someone could explain how to solve a simple CPP program such as
void swap( int & a, int & b )
{
register int t;
t = a; a = b; b = t;
}
Into MIPS.
In class we talked about frame pointers and stack pointers, offsets, and I'm just lost. How do you pass int a, and int b to the function? How do you return a value and use it in MIPS? I've seen similar problems with arrays but was hoping if someone could help me with one with just two ints to be swapped. Thanks.

Answer 1

First, you should not use the keyword register in C++, as it is deprecated and in most cases as meaningful as whitespace.
How do you pass int a, and int b to the function?
You store a and b in the registers you will use in your subroutine.
If you want to swap two values in an array, let's say at 0x80080000:
lui $t0, 0x8008 ;$t0 = 0x80080000
lw $t1 0($t0) ;load (0x80080000) in $t1
lw $t2 4($t0) ;load (0x80080004) in $t2
sw $t1 4($t0) ;store $t1 in 0x80080004
sw $t2 0($t0) ;store $t2 in 0x80080000
If you want to swap the values of two registers (xor swap algorithm):
xor $t0, $t0, $t1
xor $t1, $t0, $t1
xor $t0, $t0, $t1
Or, if you care about readability:
add $t2, $r0, $t0
add $t0, $r0, $t1
add $t1, $r0, $t2

Answer 2

You can create a function that do swap.
Something like that :
swap: # Swap function
addi $sp,$sp,-4
sw $t0, 0($sp) # Stack[0] = $t0
add $t0,$a0,$zero # $t0 = $a0
add $a0,$a1,$zero # $a0 = $a1
add $a1,$t0,$zero # $a1 = $t0 (swapping $a0 and $a1)
lw $t0,0($sp)
addi $sp,$sp,4 # $t0 = Stack[0]
jr $ra # Return to the code
And use the function Swap, like this:
li $a0,100 # $a0 = 100
li $a1,33 # $a1 = 33
jal swap # Swapping $a0 & $a1
Note: The names of the registers aren't important, you can use other.