# PICMath and SXMathMicrocontroller Bit Method

This is a clever (but confusing) trick. It only works on the PIC -- generally there are less-confusing ways to do it on other processors.

## swap w and f without a temp register

You have a value (A) in W that you want to swap with the value (B) in some file register X,

```  ; Swap (W,X):
; from Robin Abbott of Forest Electronic Developments
; currently, W contains A. X contains B.
xorwf X,w ; W := A^B; X = B
xorwf X   ; W = A^B, X := B^(A^B) = A
xorwf X,w ; W := (A^B)^A = B; X = A
; now W contains B. X contains A.
```

## move a value from X to Y without messing up W

This just builds on the above routine:

```  ; X contains A. W contains B.
movwf Y ; Y := B
movf  X,W ; W = A
; Swap (W,Y):
xorwf Y,w ; W := A^B. Y = B
xorwf Y   ; W = A^B.  Y := B^(A^B) = A
xorwf Y,w ; W := (A^B)^A = B; Y = A
; X has never been changed -- it still contains B.
; now W contains B (just as at the beginning). Y contains A.
```

Clever, but it is shorter and faster (not to mention less confusing) to use a temporary variable:

```  ; X contains A. W contains B.
movwf temp
movf  X,W ; W = A
movwf Y
movf  temp,W
; X has never been changed -- it still contains B.
; now W contains B (just as at the beginning). Y contains A.
```

## swap two variables without a temp register

If you have some value (A) in some file register X, and some value (B) in a different file register Y, and you want to swap them, you can do this:

```    ; Swap (X,Y)
; from kagato-icss-
; currently X contains A. Y contains B.
MOVF  X,W ; W:=A
XORWF Y,W ; W:=A^B
XORWF X,F ; X:=((A^B)^A)=B
XORWF Y,F ; Y:=((A^B)^B)=A
; now X contains B. Y contains A.
```

## details

Robin Abbott of Forest Electronic Developments says:

This might be of use to someone. Recently I had a project where a subroutine took a value in W and saved to a software stack:
```movwf Temp
movfw sp        ; Stack pointer
movwf FSR    ; Point to it
movfw Temp
movwf INDF
```

Trouble is it uses a temporary variable which I didn't have (it is in an interrupt). This alternative which makes use of XOR uses no temporary variable at the expense of 1 extra word:

```  movwf FSR
movfw sp
xorwf FSR
xorwf FSR,w
xorwf FSR
movwf INDF
```

I think this may be an old technique - I have vague memories of something similar from the pre-history of programming, but only found a use for

(In ____ notation that would be:

```	mov	FSR, W
mov	W, sp
xor	FSR, W
xor	W, FSR
xor	FSR, W
mov	0, W
```

)

related:

+
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