Using Auto-Initialize DMA Under Windows

• Microsoft Windows Device Driver Kit (DDK) for Windows versions 3.0 and 3.1

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 <NEAR,PASCAL,PUBLIC><si,di>

   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

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

   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)

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.