Int 21H, AH=44H, AL=0EH

3.2 and higher

GET LOGICAL DRIVE MAP

AX = 440Eh
BL = drive number (00h=default,01h=A:,etc)

CF set on error

    AX = error code (01h,0Fh) (see #01680 at AH=59h/BX=0000h)
CF clear if successful
    AL = 00h block device has only one logical drive assigned
	 1..26 the last letter used to reference the drive (1=A:,etc)

DR DOS 3.41-5.0 DRIVER.SYS does not support drive mapping and thus

  always returns AL=00h; Windows NT 4.0 also always returns AL=00h
in addition to the normal operation, if Stacker is installed, this
  call also sets the volume number at offset 58h in the Stacker
  device driver (DR DOS 3.41-5.0 only; use AX=4408h otherwise)
  (see AX=4404h"Stacker",INT 25/AX=CDCDh)

  AX=4408h,AX=440Fh,INT 2F/AX=122Bh

Text based on Ralf Brown Interrupt List Release 61