How to Build Your Own VxDCall Wrappers for Windows 95

• Microsoft Win32 Device Development Kit (DDK) version 4.0

SUMMARY

The sample found on the Microsoft Development Platform DDK CD in \DDK\BASE\SAMPLES\CVXD32 implements two methods of "wrapping" VxDCalls in the C programming language.

MORE INFORMATION

A wrapper is a friendly function call or method that hides implementation details of a difficult programming interface. The VxD in \DDK\BASE\SAMPLES\CVXD32 implements two different wrappers for submitting VxDCalls in C.

The first method uses inline functions pre-defined in \DDK\INC32\VMM.H. Here is the Get_Sys_VM_Handle example:

HVM VXDINLINE Get_Sys_VM_Handle(VOID) {

    HVM hvm;
    Touch_Register(ebx)
    VxDCall(Get_Sys_VM_Handle);
    _asm mov [hvm], ebx
    return(hvm);

Because standard C appends an underscore to the beginning of a symbol there is no conflict with the VMM_Service Get_Sys_VM_Handle.

You can also find the definitions of HVM, VXDINLINE, and Touch_Register in VMM.H file. In this way, you can use inline assembly and hide the details in an inline function in a header file. There are, however, limitations to using inline functions for your wrappers:

• They must return no more than a dword to avoid a problem with C++.
• They must have a restricted arguement list so that no parameters are used as temporary stack and all registers are preserved.

MAKE_HEADER and PREPEND are wrappers themselves to hide the implemtation of the segment structure of a VxD. In this way, arguements can be passed to and from VxD services. Notice that the Get_Sys_VM_Handle inline assembly is used to return the handle, but if more significant pre- or post-processing is required, you should implement the wrapper in Assembler.

Unlike VMM.H, the include files defining most VxD services have no predefined C wrappers for their services. Therefore, it is the responsibility of the programmer to build inline and assembly wrappers for the services needed. In this way, any VxD service is callable from C.

In VxDWraps.Inc, you can find wrapper declarations for the following groups of functions:

   VMMReg
   VMMReg
   ConfigMg
   Shell
   VxDLdr
   VComm
   IRS
   VPowerD