Question

How do I swap two variables using mips? For example:
y = 10
temp = y
y = x
x = temp

Answer 1

Do you mean MIPS architecture's instruction set? I couldn't find an instruction just for that.
You can though do it in general with 3 instructions without an additional variable:
```c
a = a ^ b;
b = a ^ b;
a = a ^ b;
```
There are similar different ways, but I like this one, I think it's simple.

In MIPS assembly (if you really need it) it should be something like that:
```
xor $t1,$t1,$t2
xor $t2,$t1,$t2
xor $t1,$t1,$t2
```
Though I haven't coded MIPS assembly so I'm not sure I did it right.

Answer 2

Yea I meant MIPS assembly. I was wondering how to swap using loading and storing if possible.

Answer 3

oh, you mean from memory? I thought you meant registers.. well in that case I'd simply read both values, and then write to the addresses:
```c
a = *p1;
b = *p2;
*p1 = b;
*p2 = a;
```
Somewhat ugly I guess, but I don't think you can do any better with memory.

Answer 4

No with registers. My attempt was something like this:

.data
.align 2
y: .space 4
x: .space 4
temp: .space 4

.text
.globl main
main:

li $t0, 10
sw $t0, y
lw $t1, x
lw $t2, temp
sw $t0, temp
sw $t1, y
lw $t3, temp
sw $t3, x

Answer 5

I don't really understand, why are you initializing only 'y' and then loading the uninitialized values of x and temp?
I'd try something like this:
```
.data
.align 2
y: .space 4
x: .space 4

.text
.globl main
main:

li $t0, 10
sw $t0, y
li $t0, 5
sw $t0, x

lw $t1, x
lw $t2, y

sw $t1, y
sw $t2, x
```

Answer 6

How do I switch only x and y after giving y a value? That is the exercise.

Answer 7

aha.. well in that case, I'll just remove the x initialization I put there
```
.data
.align 2
y: .space 4
x: .space 4

.text
.globl main
main:

li $t0, 10
sw $t0, y

lw $t1, x
lw $t2, y

sw $t1, y
sw $t2, x
```

Answer 8

Oh ok I found why I needed temp. In the data x = -10. So I need another variable to store one of the values.

Answer 9

I think I miss something. where did the -10 just come* from? O.o

Answer 10

I missed part of the given instruction. Swap variables if x = -10. so

.data
.align 2
x: .word -10
y: .space 4
temp: .space 4

and in the code you store a number like 5 into y and then swap x and y

Answer 11

so if x is not -10 you don't swap?

Answer 12

No so I'm trying to take a higher level language code and translating it to MIPS assembly.
Given:
int x = -10;
int y;
temp;

y = 5;
temp = y;
y = x;
x = temp;

Take that and make it work in MIPS.

Answer 13

first. you don't have the if(x == -10) in here. and second it's not exactly the same.
You see, writing directly in assembler allows you to use processor registers. they are separate from memory, so you can just store the value in them and then right them to opposite memory places.. you don't need temp.
The example I gave should still work if you put -10 in x up there, but I'll take a look at how to make a conditional jump

Answer 14

I don't need an if statement. I'm just trying to swap variables.

Answer 15

damn mistyped a register.. sec

Answer 16

```
.data
.align 2
y: .space 4
x: .space 4

.text
.globl main
main:

li $t0, 10
sw $t0, y

lw $t1, x
lw $t2, y

li $t0, -10
bne $t1, $t0, end

sw $t1, y
sw $t2, x

end:
```

Answer 17

But both x and y are not empty. In the case that y is given a value in the code and x already has a value. How do I swap them? I need temp as a temporary variable don't I?

Answer 18

no, because t1 will have the value of x and t2 the value of y.
if I write t1 to y, t2 still has the old value. I can write that value to x.

Answer 19

Oh I see what you mean. I can overwrite values in variables?

Answer 20

in the memory ye, because you have them stored in the registers.
If you were to try and swap between registers it would be more annoying, because after moving from one to another, you lose the value in the other..
In that case you can still do it without an extra register using the method I showed above

Answer 21

Ok so for example.
y: .word 5 //y = 5

li $t0, 10
sw $t0, y //then y = 10?

Answer 22

yes, should be, if I read the docs right. I'd try that in emulator to be sure but it makes sense =)

Answer 23

From wiki ( https://en.wikipedia.org/wiki/MIPS_instruction_set#MIPS_assembly_language ) they have short pseudo high level code for each instruction. I used it to try and understand. so in this case it has:`sw $t,C($s) -> Memory[$s + C] = $t`

Answer 24

Hm, there is one problem though, I can't find a pseudo instruction using a label. it might not be possible. let me find an emulator, i wanna check

Answer 25

K i'm using this:
http://morriswmz.jit.su/static/simple-mips-pipelined.html
Notice you have compile errors below

Answer 26

Yea but I tried that website and I don't think it can handle the MIPS I'm trying to do. It doesn't use syscall which is used when printing.

Answer 27

Lemme try, If I run out of time I'll send you a PM later, ok? :)

Answer 28

Ok thank you for helping so much :) I know there isn't much online about MIPS. I feel like I have searched everywhere.

Answer 29

Ok never mind I got an emulator to work and you were right. I can just override what was written!

Answer 30

great =)