  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.

