; in the internal RAM
DIGITS6:
     DS    6
;
;code within the ISR
;
CNTUP:
     MOV   R0, #DIGITS6
     MOV   R2, #6
CNTLP:
     INC   @R0
     CJNE  @R0, #10, CNTCP
CNTCP:
     JC    CNTDN
     MOV   @R0, #0
     INC   R0
     DJNZ  R2, CNTLP
;
CNTDN:
     ....