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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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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.
 
Parameter  Description
────────────────────────────────────────────────────────────────────────────
 
DeviceID   Identifies a virtual device.
 
Return Value
 
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.
 
Comments
 
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
 
 
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