Binding a TSR to a VxD

Note: This article assumes familiarity with the Interrupt 2FH Call-Out Interface defined by the Microsoft Windows enhanced mode. For information on the Interrupt 2FH interface, refer to Appendix C of the "Microsoft Windows Device Driver Kit: Device Driver Adaptation Guide" for Windows 3.1 and Appendix D of the "Microsoft Windows Device Development Kit Virtual Device Adaptation Guide" for Windows 3.0.

A terminate-and-stay-resident program (TSR) that loads a virtual device (VxD) when Windows enhanced mode starts must specify the fully-qualified filename to the virtual device file. This may not be convenient for end users because the virtual device file must be placed at a fixed place on the disk.

It is better to combine the TSR and the VxD together when the VxD is built because the TSR can obtain the fully-qualified filename from MS-DOS and provide the name to Windows at startup. The final product is one EXE file that holds both the TSR and the VxD. The TSR is the stub program with the VxD appended as an extended part of the EXE file. This article describes the steps necessary to perform this binding.

Please note, the binding method does not work if the TSR is in binary (COM) format rather than EXE format. The TSR filename extension (EXE or COM) is irrelevant. Therefore, while the TSR must be built in EXE format, the filename may have either the EXE or the COM extension, as desired.

The technique described here is used by the "Loadhi" program that deals with EMM drivers. However, that program is too involved to be used as an example here to demonstrate the binding process.

The following three steps must be performed to bind the TSR and VxD:

1. In the TSR, fill the SIS_Virt_Dev_File_Ptr field of the Win386_Startup_Info_Struc data structure with a pointer to a string containing the fully-qualified filename of the load file. Obtain this fully-qualified filename from the location starting at three bytes beyond the end of the environment block. A code segment resembling the following could be used:

             mov     ah, 62h
           int     21h             ; bx -> psp
           mov     es, bx
           mov     bx, 2ch         ; offset of environment block pointer
           mov     es, es:[bx]
           xor     di, di          ; es:di -> environment block
           mov     cx, -1
           xor     al, al          ; search for null's
           cld
   env_search_loop:
           repne   scasb           ; get past one null and stop
           cmp     byte ptr es:[di], 0     ; Q: end of env block?
           jnz     env_search_loop         ;   N: continue
           add     di, 3                   ;   Y: skip the last null &
                                           ;      the word before
                                           ;      the name.
           lea     si, InfoData            ; a Win386_Startup_Info_Struc
           mov     word ptr [si].SIS_Virt_Dev_File_Ptr, di
           mov     word ptr [si][2].SIS_Virt_Dev_File_Ptr, es
   						

   More information about PSP and the environment block is available in Ray Duncan's book "Advanced MS-DOS Programming" (Microsoft Press).
2. In the module definition (DEF) file for the virtual device, specify the filename for the TSR file in the STUB statement, as follows:
   STUB <TSR file name>
   For example, if the TSR is named TSRPRG.EXE, use the following line:
   STUB TSRPRG.EXE
3. In the MAKE file for the virtual device, build virtual device file with an EXE extension instead of a 386 extension. For example:

      link386 $(OBJS), VTSRPRGD.EXE $(LINKOPTIONS),,,VTSRPRGD.DEF
      addhdr VTSRPRGD.EXE