INT 2F - CD-ROM v2.10+ - SEND DEVICE DRIVER REQUEST                             
	AX = 1510h
	CX = CD-ROM drive letter (0 = A, 1 = B, etc)
	ES:BX -> CD-ROM device driver request header (see #02597 at AX=0802h)
Return: CF clear if device driver has been called (check the request header's
	      status word to determine whether an error has occurred)
	    ES:BX request header updated
	CF set if device driver has not been called
	    AX = error code (000Fh = invalid drive, 0001h = invalid function)
	    ES:BX request header unchanged
Notes:	MSCDEX initializes the device driver request header's subunit field
	  based on the drive number specified in CX
	MSCDEX v2.21 through v2.25 (at least) return error code AX=0001h if
	  nested calls are attempted
BUGS:	Novell DOS 7 NWCDEX prior to the 12/13/94 update did not initialize
	  the subunit field
	Windows95 sets CF if CX isn't a CD-ROM drive but leaves CF unchanged
	  if the drive is in fact a CD-ROM
SeeAlso: AX=0802h