The driver implementation described in this document is intended to be as general as possible, but is based on some assumptions about the properties and behavior of timer hardware.

The watchdog timer starts counting down from some settable initial value. When the timer reaches zero, it can either force the operating system to restart, or can simply shut down the system. The mechanism used to restart the system is at the discretion of the OEM configuring the server appliance. The watchdog timer operates under the assumption that as long as the operating system is running, it continually resets the timer before it can reach zero and fire. The timer device must allow the operating system to set the countdown time interval. The timer must support time intervals from 1 to 511 seconds. The timer might support smaller or larger intervals. Such additional support is not required.

The mechanism for restarting the operating system is not defined in this documentation.

Microsoft suggests that the watchdog timer hardware provide memory-mapped registers to communicate with the driver. The driver can set the start address of the I/O port or register memory when it allocates the hardware.

For more information, see Watchdog Timer Hardware Requirements for Windows Server 2003 on the Microsoft Web site.