Calling a DLL Written for Windows from a TSR for MS-DOS (67845)

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To call the TSR program, an application can use the DPMI Simulate Real Mode Interrupt function or the DPMI Call Real Mode Procedure function with a FAR return frame.

To call a DLL from a TSR, perform the following four steps:

1. Design the function to be called from the TSR similar to an interrupt service routine (ISR), and put it into a DLL. The module definition (DEF) file for the DLL must contain the statement CODE PRELOAD FIXED. Here is a code fragment for a callback function:

       ; My callback code in a CODE PRELOAD FIXED Windows DLL
      ;
      ; Note:  The real mode CS:IP in the real mode CallStruct must
      ;        be fixed up for IRET to work correctly, as shown below:
   
      My_Call_Back:                         ; The real mode callback entry
   
         ;==========================================================
         ; On entry to this routine:
         ;
         ;   DS:SI -> Real mode SS:SP
         ;   ES:DI -> Real mode call structure
         ;   Interrupts disabled
         ;
         ; On exit:
         ;   ES:DI -> Real mode call structure
         ;==========================================================
   
         call     Do_It                     ; Function that handles
                                            ; the callback
   
         cld
         lodsw                              ; Get real mode IP from stack
         mov      WORD PTR es:[di+2Ah], ax  ; Store IP in call struc
         lodsw                              ; Get real mode CS from stack
         mov      WORD PTR es:[di+2Ch], ax  ; Store CS in call struc
         add      WORD PTR es:[di+2Eh], 4   ; Bump real mode SP past CS:IP
   
         iret                               ; Exit My_Call_Back
   						

2. Use the DPMI Allocate Real Mode callback function to set up a callback address to the function, as follows:

        mov      ax, 0303h               ; Allocate real mode callback
                                          ; address
         push     ds
         push     cs
         pop      ds
         mov      si, OFFSET My_Call_Back
         pop      es
         mov      di, OFFSET MyRMCS       ; Offset of real mode call
                                          ;   structure, used for DPMI
                                          ;   translation services
                                          ;   (see Chapter 11 of the
                                          ;   INTEL DPMI Spec. v 0.9)
   
         int      31h                     ; Call DPMI
   
         jc       CB_Error                ; If carry set, call failed
         jmp      Set_CB_Addr
   
      CB_Error:
         mov      ax, 1
         jmp      Set_CB_Exit
   
      Set_CB_Addr:
         mov      WORD PTR CB_Addr+2, cx   ; Store SEG:OFF of callback
         mov      WORD PTR CB_Addr, dx     ;   address in local variable
   
      Set_CB_Exit :                        ; Exit
   						

3. Pass the real mode callback address, returned from the code above, to the TSR and store it, as follows:

      ; Call the TSR, passing the real mode callback in BX:CX
   
      Notify_TSR:
   
         mov      bx, WORD PTR CB_Addr+2    ; Real mode callback seg
         mov      cx, WORD PTR CB_Addr      ; Real mode callback off
         mov      ax, 8F00h                 ; ID and function number
         int      TSR_Int_No
   
         cmp      al, 00h
         jz       Notify_Success
   
      Notify_Failure:
         mov      al, 1                     ; Return failure in AL
   
      Notify_Success:
   						

4. When it is time for the TSR to call the DLL, use Interrupt 2Fh to determine whether the current virtual machine (VM) is VM 1 (the Windows VM). If it is, use the assembly language statement CALL DWORD PTR dwCallBackAddr to call the real mode callback address. If the current VM is not VM 1, use the Interrupt 2Fh service Switch VMs and Callback (Function 1685h). Specify the real mode callback address stored in the TSR in step 3 above for the ES:BX value in the call.

      ;
      ; Code in TSR, which is loaded before Windows
      ;
   
      Call_DLL_Entry:
   
         mov      ax, 1683h                 ; Windows Interrupt 2Fh
         int      2Fh                       ;    (Get Current VM)
   
   
         cmp      bx, 1                     ; Windows is always VM 1
                                            ; (version 3.0 *ONLY*)
         jz       Call_DLL                  ; We're in Windows VM
   
      Switch_VMs_And_Call_DLL:              ; We're not in Windows VM
   
         mov      ax, 1685h                 ; Windows Interrupt 2Fh
                                            ;   switch VMs and callback
         mov      bx, 1                     ; Windows 3.0 is always VM 1
         mov      cx, 0                     ; All flags off
         mov      dx, 0                     ; Priority boost not required
         mov      si, 0                     ;
         mov      di, seg SwitchProc        ; Segment of switch procedure
         mov      es, di                    ;
         mov      di, offset SwitchProc     ; Offset of switch procedure
         int      2Fh                       ;
   
         jnc      My_1685h_Success
   
      My_1685h_Error:
   
         ; AX = error code:   1 = Invalid VM ID
         ;                    2 = Invalid priority boost
         ;                    3 = Invalid flags
   
         .
         .
         .
   
         jmp      Call_DLL_Exit
   
      My_1685h_Success:
   
           mov     ax, 0
           jmp     Call_DLL_Exit
   
      Call_DLL:
   
         Call     DWORD PTR CB_Addr          ; FAR CALL to real mode
                                             ;   callback
   
      Call_DLL_Exit:
   
         .
         .
         .
   
         iret                               ; return to caller or
                                            ;   hardware interrupt
   
      SwitchProc proc
   
         pusha
         push    ds
         push    es
   
         call    DWORD PTR CB_Addr          ; FAR CALL to real mode
                                            ;   callback
         pop     es
         pop     ds
         popa
   
         iret                               ; Switches back to VM from
                                            ;   which it was called
   
      SwitchProc endp
   						

The four steps above allocate a unique procedure callback address and pass it to the TSR, which stores the address. When the TSR calls the DLL, if the Windows virtual machine (VM) is running, the TSR calls the callback address directly. Otherwise, the TSR calls the Switch VMs and Callback function to schedule the Windows virtual machine. One parameter for this function is the address to call once the requested VM is running.

In this case, the Windows scheduler switches VMs, then it calls the SwitchProc function in the TSR. When SwitchProc calls the routine in the DLL that corresponds to the allocated real mode callback address, the DLL receives control to perform its processing.

Once the DLL completes its processing, it executes an IRET instruction to return control to the SwitchProc. Then SwitchProc itself executes an IRET instruction. At this point, the Windows scheduler switches back to the VM from which it was called.

All Interrupt 2Fh functions are documented in Appendix C of the "Microsoft Windows Device Driver Kit: Device Driver Adaptation Guide" for Windows 3.1 and in Appendix D of the "Microsoft Windows Device Development Kit Virtual Device Adaptation Guide" for Windows 3.0.

DPMI (Interrupt 31h) services are documented in the Intel document "MS-DOS Protected Mode Interface Specification." A paper copy of the DPMI specification is available free of charge from Intel Literature. In the United States, contact Intel at (800) 548-4725. Outside the US, contact the Intel distributor for your country.