The kernel's callback mechanism provides a general way for drivers to request and provide notification when certain conditions are satisfied.
A driver can create a callback object, and other drivers can request notification for conditions associated with this driver-defined callback. In addition, the system defines two callback objects for driver use.
Every callback object has a name and a set of attributes, defined when the object is created. The system-defined callback objects are named \Callback\SetSystemTime, \Callback\PowerState, and \Callback\ProcessorAdd; driver-defined callbacks must not duplicate these names.
To request notification from a system- or driver-defined callback, a driver opens the callback object and registers a callback routine. When the conditions defined for the callback become true, its creator triggers notification. In turn, the system calls all the callback routines registered for the callback.
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