Port 040  -  8253-5 Channel 0

        Used for system timer INT 8 via 8259 IRQ 0. Uses a divisor of
        0 (65336) to cause an interrupt 18.20648193 times per second,
        or 54.925493 ms apart. The disk motor timing is also based
        on INT 8. Operates in mode 3 (see port 043). In its normal mode
        of operation, a pseudorandom number can be obtained this way:

                        MOV    AL,0
                        OUT    AL,43h
                        IN     40h,AL
                        MOV    AH,AL
                        IN     40h,AL
                 16-bit random number is now in AX.

        This latching does not affect its countdown.