Microsoft KB Archive/72814

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Windows 3.0 Does Not Support Auto-Initialize DMA

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Q72814

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, version 3.0

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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

EndProc XDMA_Device_Init 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

XDMA_Not_Our_Channel:

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

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

VxD_DATA_SEG

Old_VDMAD_Mask_Channel dd      0
 * A place to store the far pointer to the old VDMAD service for
 * masking the DMA channel -- it must be called!

XDMA_Channel           dd      0
 * The channel (handle) on which this VxD will allow Auto-DMA to
 * occur.

XDMA_Mode              db      0
 * The mode that the application has requested the DMA channel to be
 * in -- this may or may not be Auto-DMA mode.

VxD_DATA_ENDS 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 &quot;export&quot; 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

BeginProc XDMA_Protect_Mode_API

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.

XDMA_PM_API_Fail:

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

or     [ebp.Client_Flags], CF_Mask ret

XDMA_PM_API_Exit:

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

and    [ebp.Client_Flags], NOT CF_Mask ret

EndProc XDMA_Protect_Mode_API The XDMA_Set_Mode function must allow the calling application to set the state of the &quot;auto-init&quot; 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

EndProc XDMA_Set_Mode 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!

EndProc XDMA_Set_Channel 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...

XDMA_Handler_Exit:

ret

EndProc XDMA_Handler 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 &quot;Microsoft Windows Device Development Kit Virtual Device Adaptation Guide&quot; 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

.       .        . If the above code succeeds, then the VXDMAD's API can be called by the following code:

       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! If the above two calls succeed, the application can use auto- initialize mode. The application must execute an OUT instruction to set the physical mode of the channel -- WITH THE AUTO-INIT BIT CLEARED! VXDMAD sets the Auto-Initialize bit at the right time. The following information is from RichP in response to a customer needing to use Loop-mode (a/k/a Auto-Initialize mode) DMA (NeilSa had the SR; the SR number is G900612-110):

In 3.0 this auto-initialize mode isn't supported directly. The problem involves trying to track ownership of the DMA transfer. VDMAD was designed for per-VM DMA transfers, but the nature of this device sounds like it is global DMA. It would be possible for them to write a very small and simple VxD which calls VDMAD to virtualize their DMA channels. VDMAD provides services for additional VxD's to handle special types of DMA. I have already helped an OEM write a similar device for a network adapter which uses global DMA. This is the only proposed work-around at this time.

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