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.
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.
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.
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.
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
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