When an application calls DeviceIoControl, the system calls the control procedure of the VxD identified by the given device handle. The EAX register contains the W32_DEVICEIOCONTROL message, and the ESI register contains the address of a DIOCParams structure. The structure contains all of the parameters that the application specified in the DeviceIoControl function as well as additional information. The VxD should examine the dwIoControlCode member of the DIOCParams structure to determine the action to perform. The lpvInBuffer member contains supporting data that the VxD needs to complete the action. After processing the control code, the VxD should copy any information that it needs to return to the application to the buffer specified by the lpvOutBuffer member.
If the VxD successfully processes the control code, it should clear the EAX register before returning. Otherwise, the VxD should set EAX to a nonzero value.