A VxD can determine whether an application has requested an asynchronous operation by checking the lpoOverlapped member of the DIOCParams structure. If lpoOverlapped specifies the address of an OVERLAPPED structure, the application has requested an asynchronous operation. In that case, the VxD should return – 1 in the EAX register and then process the specified control code.
The operating system does not take any steps to make the application's memory available to the VxD at all times (in all contexts) for asynchronous operations. When implementing asynchronous operations, the VxD must use the appropriate virtual machine manager (VMM) services, such as LinPageLock with the PAGEMAPGLOBAL value, to make the application's memory pages available across contexts, including access to the OVERLAPPED structure.
When the VxD finishes processing the control code, it must notify the application by calling the VWIN32_DIOCCompletionRoutine service provided by VWIN32.VXD. The EBX register must contain the value of the Internal member of the OVERLAPPED structure. This member is reserved for operating system use only. Also, if the VxD copies any data to the buffer specified by the lpvOutBuffer member of the DIOCParams structure, the VxD must specify the count of bytes copied to the buffer in the InternalHigh member of OVERLAPPED. The remaining members of overlapped, Offset and OffsetHigh, can be used for developer-defined data.
Essentially, the VWIN32_DIOCCompletionRoutine service helps signal the event identified by the hEvent member of the OVERLAPPED structure. The application monitors the event to determine when the asynchronous operation is completed.