Article: Q72814
Product(s): Microsoft Windows Device Driver Kit
Version(s): 3.0
Operating System(s):
Keyword(s):
Last Modified: 03-NOV-1999
-------------------------------------------------------------------------------
The information in this article applies to:
- Microsoft Windows Device Development 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
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
;; 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
;; The channel (handle) on which this VxD will allow Auto-DMA to
;; occur.
XDMA_Channel dd 0
;; The mode that the application has requested the DMA channel to be
;; in -- this may or may not be Auto-DMA mode.
XDMA_Mode db 0
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 "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
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 "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
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 "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
.
.
.
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.
Additional query words: 3.00
======================================================================
Keywords :
Technology : kbAudDeveloper kbWin3xSearch kbWinDDKSearch kbWinDDK300
Version : :3.0
=============================================================================