System Hung (Windows default)
The watchdog timer is used to detect a system problem, such
as software in an endless loop (Ed. or Windows in it's normal state), with
interrupts disabled. The watchdog works by checking that the system timer
0 is being serviced regularily. If the watchdog detects that IRQ0 is not
being serviced quick enough (ie. lost timer ticks), the timer overflow
initiates a Non-Maskable Interrupt (NMI).
This timer only operates in Mode 0, and only has an 8
Each trigger of IRQ 0 will initiate the interrupt 8 timer
service routine. At the completion of the timer service routine, the IRQ
0 line is cleared. This same line is fed to timer 3's gate. Since no clock
occured from timer 0 while timer 3's gate was active, timer 3 remains in
the same state.
(Windows Normal State)
IRQ 0 is not serviced and the IRQ 0 active line remains high.
The next clock from timer 0 output is allowed to clock timer 3 down. Once
the pre-set count is exhausted, timer 3's output goes high, causing an
NMI. The BIOS NMI service handler can then take appropriate action such
as resetting the system. (Ed or maybe DEL C:|WINDOWS?)
The output of timer 3 can be read from port 92h and examining
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