Microsoft KB Archive/83012

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Using Auto-Initialize DMA Under Windows

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Q83012

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.0, 3.1

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SUMMARY
VADMAD, the Virtual Auto-Initialize Direct Memory Access Device, is provided with the Windows Multimedia Device Development Kit version 1.0, and the Multimedia Extensions to Windows version 1.0, to allow devices that use auto-initialize DMA to function properly under Windows 3.0.

However, under Windows version 3.1, the VDMAD (Virtual DMA Device) supports an auto-initialize DMA transfer, without using VADMAD.

Do not use VADMAD with Windows 3.1 because it conflicts with VDMAD.

The remainder of this article describes how an application can perform auto- initialize DMA transfers under Windows 3.0 and under Windows 3.1.

MORE INFORMATION
Under Windows version 3.1, auto-initialize DMA is performed using the same method as under MS-DOS: set the auto-initialize bit in the mode register and perform the transfer.

Under Windows 3.0, the application must call VADMAD to set the auto- initialize bit of the DMA mode register.

If the VADMAD driver is installed, the GetVADMADEntry function, listed below, returns its address in ES:DI. If INT 2Fh returns 0 in ES, VADMAD is not installed, and attempts to call the address will cause a general protection fault (GP fault).

VADMAD_Entry dd ?

cProc GetVADMADEntry  xor di,di                         ; Clear DI   mov  es,di                         ; Clear ES   mov  ax,1684h                      ; Get device entry mov bx,0444h                      ; VADMAD Device int 2f                            ; Get function entry mov word ptr [VADMAD_ENTRY],di    ; Save entry point mov word ptr [VADMAD_ENTRY+2],es

cEnd Once the VADMAD address is available, set the DMA mode using the following code:

  push dx                   ; Save DX   mov  dx,VADMAD_Service    ; Set VADMAD operation call VADMAD_Entry        ; Set mode or channel out 0Bh,al               ; Out to mode register pop dx                   ; Restore DX The values for the VADMAD_Service are defined as follows:

0: Set the VADMAD mode (put the desired mode into the AX register)

1: Set the VADMAD channel (put the desired channel into the AX register) Two calls to the VADMAD_Entry are required: one to set the mode and one to set the channel.

Under Windows 3.1, set the mode by modifying the mode register, as follows:

  mov  al,01010100b   ; Auto-Initialize, Write, Channel 0 out 0bh,al         ; Send the data to the DMA mode register. An application can use the same procedure to program channels 4 through 7, which are 16-bit channels; channels 0 through 3 are 8-bit channels. To program channels 4 through 7, use port D6h rather than 0Bh.

Additional query words: 3.00 3.10

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