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:mov Temp, W mov W, sp ; Stack pointer mov FSR, W ; Point to it mov W, Temp mov 0, W
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:mov FSR, W mov W, sp xor FSR, W xor W, FSR xor FSR, W mov 0, W
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.mov W, x ; Get X xor y, W ; Y is now X^Y xor W, y ; W is now (X^y)^X==Y (say OldY) mov x, W ; Now X is OldY xor y, W ; 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
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<A HREF="http://www.massmind.org/Techref/scenix/lib/math/bit/swap_sx.htm"> SX Microcontroller Bit Math Method - swap w and f without a temp register</A>
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