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

3.2 and higher

SET LOGICAL DRIVE MAP

AX = 440Fh
BL = physical 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
    drive now responds to next logical drive number

maps logical drives to physical drives, similar to DOS's treatment of

  a single physical floppy drive as both A: and B:
DR DOS 3.41-5.0 DRIVER.SYS does not support drive mapping and thus
  always returns an error on this function

  AX=440Eh,INT 2F/AX=122Bh

Text based on Ralf Brown Interrupt List Release 61