Weekend Quiz: What does the following snippet do?

;inputs: r5, r6, r7
;output: a

AaaOfBbbb:
        mov   a,r6        ;if Ccccc is 1 or 2
        cjne  a,#3,AaaOfBbbbX1
AaaOfBbbbX1:
        mov   a,r7        ;   count it as if it would be previous Dddd
        subb  a,#0
        mov   b,a
        rr    a
        rr    a
        anl   a,#03Fh     ;subtract the correction
        clr   c
        subb  a,b         ;   and store as negative
        mov   b,a
        mov   a,r7        ;Now the calculation itself: Dddd*2
        rl    a
        add   a,b         ;plus the precalculated corrections
        mov   b,a
        mov   a,r6        ;plus sum of Aaa-s for Ccccc 
        add   a,#AaaOfBbbbTab-AaaOfBbbbX2-1   
        movc  a,@a+pc     ;   mod 7, picked from table
AaaOfBbbbX2:
        add   a,r5        ;plus Aaa
        add   a,b
        add   a,#4        ;plus correction for first Aaa of Dddd
        mov   b,#7        
        div   ab          ;sum mod 7 is the result
        mov   a,b
        ret

AaaOfBbbbTab:
        db    0,3,4,0,2,5,0,3,6,1,4,6        

Enjoy! :-)

JW