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.
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