Compatibility: Win 3.x
Expects: AX    1684H
BX    Device ID (see VxD IDs)
ES:DI 0000:0000 (required for pre-3.0 Windows)
Returns: ES:DI device entry-point address
0000:0000 if that device does not support entry-point calls
Info: TSRs and DOS-level device drivers may use this function as a way
to communicate with a Windows virtual device driver (VxD).

This is normally used to access "published" APIs supplied by
software vendors or it can be called by a TSR which has loaded a
windows virtual device (and therefore knows its ID).

Windows virtual device drivers can use this to provide various
services to DOS device drivers and TSRs.  To access any such
services, set registers as needed by the device, and use a FAR
CALL to execute the code at ES:DI.

The INT 2fH 1607H (device call out) service provides another
means of communication.

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INT 2fH 1684H: Get Device Entry-Point Address (for VxDs)