Int 21H, AH=33H, AL=05H

4.0 and higher

GET BOOT DRIVE

    AX = 3305h

DL = boot drive (1=A:,…)

This function does not use any of the DOS-internal stacks and may
         thus be called at any time.  It is directly dispatched from
         the INT 21h entry point with interrupts disabled.
      NEC 9800-series PCs always call the boot drive A: and assign the other
        drive letters sequentially to the other drives in the system
      this call is supported by OS/2 Warp 3.0, but not earlier versions of
        OS/2; it is also supported by Novell DOS 7

Text based on Ralf Brown Interrupt List Release 61