--- /dev/null
+ APC
+
+ Asynchronous procedure call
+
+ An APC is a Kernel-defined control object representing a procedure
+ that is called asynchronously. APCs are thread-context dependent; that
+ is, they are queued to a particular thread for execution.
+
+ There are three different kinds of APCs in NT:
+
+ User APCs are used by certain asynchronous NT system services to allow
+ user-mode applications or protected subsystems to synchronize the
+ execution of a thread with the completion of an operation or the
+ occurrence of an event such as a timers expiration. User APCs are, by
+ default, disabled. That is, they are queued to the user-mode thread,
+ but they are not executed except at well-defined points in the
+ program. Specifically, they can only be executed when an application
+ or protected subsystem has called a wait service and has enabled
+ alerts to occur, or if it has called the test-alert service.
+
+ Kernel APCs are normal kernel-mode APCs. They are much like a normal
+ user APC except that they are executable by default. That is, they are
+ enabled except when the thread is already executing a Kernel APC.
+ (Note that a special Kernel APC always preempts these.)
+
+ Special Kernel APCs cannot be blocked except by running at a raised
+ IRQL. They are executed at APC_LEVEL IRQL (see IDT), in kernel mode.
+ These types of APCs are used by the system to force a thread to
+ execute a procedure in the threads context. An example of this is I/O
+ completion: the I/O Manager needs to get back into the context of the
+ original requestor of the I/O operation so that it can copy buffers,
+ and so forth. In order to do this, the I/O Manager must be able to
+ access the virtual address space of the thread/process, and the most
+ efficient way to complete the operation is to be in the calling
+ threads context.
+
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