serisr:
        push ACC        Save registers on stack
        push DPL
        push DPH
        jnb RI,Check_tx We did not receive anything.  
        clr RI
        mov A,SBUF      Get received char in ACCU
        mov DPH,#80h    Buffer is at 08000H, initialise DPH to point at buffer
        mov DPL,08h     DPL is kept in 08h
        movx @DPTR,A    Save received char in buffer
        inc 08h         Bump pointer
        setb 20h.4      ??????
Check_tx:
        jnb TI,exit_isr Did we transmit a char?
        setb 20h.3      yes, set bit ?????
        clr TI
exit_isr:
        pop DPH         All done, restore registers
        pop DPL
        pop ACC
        reti            Exit ISR