Windows 3.0 Does Not Support Auto-Initialize DMA

• Microsoft Windows Device Driver Kit (DDK) for Windows version 3.0

SUMMARY

In version 3.0 of the Microsoft Windows graphical environment, enhanced mode does not support the Auto-Initialize mode of the 8237 direct memory access (DMA) controller. This is due to a limitation of the virtual DMA device (VDMAD). This article discusses one possible method (short of modifying VDMAD) of partially supporting the Auto-Initialize mode.

MORE INFORMATION

The VDMAD does not support the Auto-Initialize mode of the 8237 DMA controller. If an application attempts to program the DMA controller in Auto-Initialize mode, VDMAD terminates the application.

To work around this behavior, it is necessary to write a small virtual device driver (VxD) that hooks the VDMAD_Mask_Channel service, and provides a replacement service. (In the following code, this VxD is referred to as VXDMAD.)

Please note that the following code assumes that all operations will be in the system virtual machine (the Windows VM) and that only one DMA channel uses Auto-DMA at a time. This functionality could be extended circumstances require it.

To begin, VXDMAD must hook the VDMAD_Mask_Channel service during Phase 2 of the protected mode initialization (Device_Init):

BeginProc XDMA_Device_Init

        mov     eax, VDMAD_Mask_Channel
        mov     esi, OFFSET32 XDMA_Mask_Channel
        VMMcall Hook_Device_Service
        mov     [Old_VDMAD_Mask_Channel], esi
        clc
        ret

In VXDMAD's XDMA_Mask_Channel service, if the channel is owned by VXDMAD, ignore the call to mask the DMA channel. See below for how VXDMAD retrieves the mode and channel to look for.

BeginProc XDMA_Mask_Channel

        pushfd

        cmp     eax, [XDMA_Channel]         ; Is it our channel??
        jne     XDMA_Not_Our_Channel        ; No. Let VDMAD mask it.

        test    [XDMA_Mode], DMA_AutoInit   ; Is it auto-init?
        jz      XDMA_Not_Our_Channel        ; No. Let VDMAD mask it.

        popfd                               ; Ours. Do nothing!
        ret

        popfd                               ; Of course.
        jmp     [Old_VDMA_Mask_Channel]     ; Let old service take it.

To make things clear, the data segment for this fictitious VxD should be defined as follows:

VxD_DATA_SEG

;; A place to store the far pointer to the old VDMAD service for ;; masking the DMA channel -- it must be called!

Old_VDMAD_Mask_Channel  dd      0

XDMA_Channel            dd      0

XDMA_Mode               db      0

This brings us to the next thing VXDMAD must do. VXDMAD checks the channel and mode to ensure that it should not let the VDMAD mask the channel. VXDMAD can perform the check only if VXDMAD knows the channel and mode the application uses. Therefore, VXDMAD must also implement a communications method for the application to tell this VXDMAD the channel and mode to monitor. The easiest way to provide a communications method is to use the one API that a VxD can "export" to applications. For this example, VXDMAD only needs a protected mode API; however, there is no reason why this could not be done for the real mode API.

The two functions VXDMAD must implement are: one to set the channel to allow Auto-DMA, and one to set the mode for that channel. These functions are implemented as follows:

XDMA_API_SET_CHANNEL    equ     0
XDMA_API_SET_MODE       equ     1

        movzx   eax, [ebp.Client_AH]        ; Client AH = func number
        cmp     eax, XDMA_API_SET_CHANNEL   ; Set the channel?
        jz      XDMA_Set_Channel            ; If so, do it.

        cmp     eax, XDMA_API_SET_MODE      ; Set the mode?
        jz      XDMA_Set_Mode               ; If so, do it.

        ;;  This is a failed exit -- set client's carry and return

        or      [ebp.Client_Flags], CF_Mask
        ret

        ;;  This is a successful exit -- clear client's
        ;;  carry and return

        and     [ebp.Client_Flags], NOT CF_Mask
        ret

The XDMA_Set_Mode function must allow the calling application to set the state of the "auto-init" mode bit used on DMA transfers. This function only tells VXDMAD if the DMA channel (set by XDMA_Set_Channel) is in auto-init mode. The application still needs to execute the OUT instruction to the mode port, with the auto-init bit cleared, to set the physical DMA mode and have that setting reflected in the VM's virtual state, which is held by the VDMAD that traps the port.

BeginProc XDMA_Set_Mode

        VMMcall Test_Sys_VM_Handle      ; Are we in the system VM?
        jnz     XDMA_PM_API_Fail        ; No. Set carry and ret

        mov     al, [ebp.Client_AL]     ; AL = mode
        and     al, DMA_AutoInit        ; Only want auto-init bit
        mov     [XDMA_Mode], al         ; Save it

        jmp     XDMA_PM_API_Exit        ; Return carry clear

The XDMA_Set_Channel function virtualizes the channel specified (in AL) so that VXDMAD can take control when the port (physical) mode is set.

BeginProc XDMA_Set_Channel

        xor     eax, eax                    ; Clear eax
        cmp     [XDMA_Channel], eax         ; Are we handling a port?
        jne     XDMA_PM_API_Fail            ; Yep! Fail one at a time!

        VMMcall Get_Sys_VM_Handle           ; EBX = system VM handle
        mov     al, [ebp.Client_AL]         ; Get channel # to
                                            ;   virtualize
        mov     esi, OFFSET32 XDMA_Handler  ; Handler for this channel
        VxDcall VDMAD_Virtualize_Channel    ; Attempt virtualization
        jc      XDMA_PM_API_Fail            ; Fail! Set carry and ret

        mov     [XDMA_Channel], eax         ; Save handle
        jmp     XDMA_PM_API_Exit            ; Return success!

The only function remaining is the handler for the DMA channel. This handler is called every time the physical state of the DMA channel that VXDMAD is virtualizing changes. This will force the DMA_AutoInit bit to be set if the application has requested that it be set (using XDMA_API_SET_MODE and XDMA_API_SET_CHANNEL).

BeginProc XDMA_Handler

        VMMcall Test_Sys_VM_Handle      ; We in system VM?
        jnz     XDMA_Handler_Exit       ; No! Exit...

        VxDcall VDMAD_Get_Virt_State    ; Need to set mode bit
        and     dl, NOT DMA_AutoInit    ; Clear if set (safety)
        or      dl, [XDMA_Mode]         ; Set auto-init if we need to
        VxDcall VDMAD_Set_Virt_State    ; Tell VDMAD about it

        VxDcall VDMAD_Default_Handler   ; Continue with VDMAD...

        ret

Once the VXDMAD VxD is installed (from the SYSTEM.INI file), an application must contain code similar to the following to use its services (the Interrupt 2Fh functions are documented Appendix D of the "Microsoft Windows Device Development Kit Virtual Device Adaptation Guide" for Windows 3.0):

VXDMAD_API      dd      0

        .
        .
        .

        mov     ax, 1600h               ; Enhanced mode check
        int     2Fh                     ; Shazam!
        test    al, 7Fh                 ; Is enhanced mode running?
        jz      not_running_enhanced    ; Can't use VXDMAD --
                                        ;   Not there!

        mov     ax, 1684h               ; Get device API pointer
        mov     bx, VXDMAD_Dev_ID       ;   for the VXDMAD VxD
        int     2Fh                     ; Get it
        mov     word ptr VXDMAD_API, di ;   and save the address
        mov     word ptr VXDMAD_API+2, es

        mov     ax, es                  ; Is VXDMAD installed?
        or      ax, di
        jz      vxd_not_installed       ; If not, can't use it

        .
        .
        .

        mov     ah, XDMA_API_SET_CHANNEL
        mov     al, Channel_To_Virtualize
        call    VXDMAD_API
        jc      This_Call_Failed        ; Carry set if failure!
        mov     ah, XDMA_API_SET_MODE
        mov     al, Mode_For_Channel    ; With auto-init bit set or
                                        ;   clear
        call    VXDMAD_API
        jc      This_Call_Failed        ; Carry set if failed!