Interrupt 2Fh Function 1684h

mov     bx, [DeviceID]  ; Device identifier

mov     ax, 1684h       ; Get Device Entry Point Address
int     2Fh             ; multiplex interrupt

mov     word ptr [DevAddr], di
mov     word ptr [DevAddr+2], es    ; es:di contains entry point address

Get Device Entry Point Address (Interrupt 2Fh Function 1684h) retrieves the entry point address for a virtual device's service functions. MS-DOS device drivers or TSRs typically use this function to communicate with virtual devices they have explicitly loaded.

DeviceID

Identifies a virtual device.

• The return value is the entry-point address contained in the ES:DI register pair if the function is supported. Otherwise, ES:DI contain zero.

Any virtual device can provide service functions to be used by MS-DOS programs. For example, the virtual-display device provides services that the Windows old application program uses to display MS-DOS programs in a window. It is the responsibility of the MS-DOS program to provide the appropriate virtual-device identifier. The function returns a valid address if the virtual device supports the entry point.

MS-DOS programs call the entry point using a far call instruction. The services provided by the virtual device depend on the device. It is the responsibility of the MS-DOS program to set registers to values that are appropriate to the specific virtual device.

For versions of Windows prior to version 3.0, the program must set the ES:DI register pair to zero before calling this function.

Example

The following retrieves the entry point address for the virtual device identified by My_Device_ID:

xor     di, di              ; set es:di to zero for version 2.x
mov     es, di
mov     bx, My_Device_ID

mov     ax, 1684h
int     2Fh

mov     ax, es
or      ax, di
jz      API_Is_Not_Supported