INT 21 - Stacker - GET DEVICE DRIVER ADDRESS AND SET VOLUME NUMBER
AX = 4404h
BL = drive number (00h = default, 01h = A:, etc)
CX = 0004h
DS:DX -> DWORD buffer to receive device driver address
Note:   In addition to returning the address of the Stacker device driver,
this call also sets the volume number at offset 3Eh in the device
driver (see INT 25/AX=CDCDh)

See also: 25CDCD

214404 - Stacker - GET DEVICE DRIVER ADDRESS AND SET VOLUME NUMBER