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DEVICE CONTROL PROCEDURE

Every virtual device needs a device control procedure. The VMM calls this procedure to 
send the virtual device system control messages. The system control messages direct
the virtaul device to carry out actions, such as initializing itself, and to notify
the virtual
device of changes to virtual machines, such as a virtual machine is being created. Most
virtual devices define the device control procedure by using the
Begin_Control_Dispatch, Control_Dispatch, and End_Control_Dispatch macros as
in the following example:
        Begin_Control_Dispatch VSAMPLED
                Control_Dispatch Sys_CriticalInit, VSAMPLED_Crit_Init
                Control_Dispatch Device_Init, VSAMPLED_Device_Init
                Control_Dispatch Sys_Critical_Exit, VSAMPLED_Crit_Exit
        End_Control_Dispatch VSAMPLED

In this example, the macros create a device control procedure, named
VSAMPLED_Control, and generate instructions that check for the messages
Sys_Critical_Init, Device_Init, and Sys_Critical_Exit. When these messages are sent
to the procedure, the procedure passes control to a corresponding procedure, such as
VSAMPLED_Device_Init, to process the messages. These message-processing
procedures must be defined by the virtual device.

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