Dear Kai Klaas
Can I have the audacity of trying to 'comment' your code?

;;;;;;;;;;;;;;;;;;;;;;;;;;;A was amsn alsn
SWAP A ;Alsn<->Amsn ;A becomes alsn amsn

;;;;;;;;;;;;;;;;;;;;;;;;;;;R0 was rmsn rlsn
XCHD A,@R0 ;Alsn<->R0lsn ;R0 becomes rmsn amsn
;;;;;;;;;;;;;;;;;;;;;;;;;;;A becomes alsn rlsn

XCH A,@R0 ;A<->R0 ;A becomes rmsn amsn
;;;;;;;;;;;;;;;;;;;;;;;;;;;R0 becomes alsn rlsn
P.S. msn means most significant nibble and lsn means least ....
A.S.Rudra