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:

subroutine:

movwf Temp
movfw sp        ; Stack pointer
movwf FSR    ; Point to it
movfw Temp
movwf 0

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:

subroutine:

  movwf FSR
  movfw sp
  xorwf FSR
  xorwf FSR,w
  xorwf FSR
  movwf 0

You can also use this to swap two variables (say x and y) without a
temporary variable leaving X (or Y if order is reversed) in W.

    movfw x       ; Get X
    xorwf y         ; Y is now X^Y
    xorwf y,w     ; W is now (X^y)^X==Y  (say OldY)
    movwf x       ; Now X is OldY
    xorwf y         ; finally Y is (OldX^Y)^Y==OldX

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 it now
!


Robin Abbott - robin.abbott@fored.co.uk

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