dec  losec     ; decrease seconds (units)
     mov  a,losec   ; get the value to a for testing
     cpl  a         ; If it was 0ffh, it will be 0 now
     jnz  ....      ; Jump away if you didn't underflow

     mov  losec,#9  ; reload seconds (units) with 9
     dec  hisec     ; decrease seconds (tens)
     ...