How do you make a byte stack

Development Boards MicroMod
1

I am monitoring the NMEA stream from a GPS and need to recognize the $GPGLL header. The scheme I am trying use is to take bytes out of the UART and shift them up through three registers. After each new entry I test for the header pattern.

The problem is that nothing gets shifted through the registers. I use the SWPB instruction elsewhere in my code and it works. Something is wrong with my shift algorithm but I can’t see it. Any ideas?

;-----------------------------------------------------------------------------
;   Character stream - every character recieved is entered into a shift 
;   register that consists of R5 (lo order) through R7 (hi order)
;-----------------------------------------------------------------------------

Char:       swpb    R7     ; Shift byte 5 to the byte 6 position
            swpb    R6     ; Move byte 3 to the byte 4 position
            mov.b   R6, R7 ; Move byte 4 to the byte 5 position
            swpb    R5      ; Move byte 1 to the byte 2 position
            mov.b   R5, R6  ; Move byte 2 to the byte 3 position
            mov.b   R12, R5   ; Insert new byte into position 1
            ret