INT 15 - SYSTEM - ENABLE/DISABLE WATCHDOG TIMEOUT (PS50+)
        AH = C3h
        AL = function
            00h disable PS/2 watchdog timer
            01h enable PS/2 watchdog timer
                BX = timer counter (0001h-00FFh)
            02h disable Gearbox system
            03h enable Gearbox system
Return: CF set on error
        CF clear if successful
Note:   the watchdog timer generates an NMI

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