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  In DOS versions prior to 4.0, function 33H (decimal 51) can be used only
  to examine or update the DOS internal flag that controls Ctrl-C checking.
  (See Chapter 17.) DOS version 4 supports a third subfunction that returns
  the drive ID of the disk drive used to boot the system.

  To determine the boot drive ID, call function 33H with AL = 5. DOS version
  4 returns the drive ID in register DL. Drive ID values can be 1 (drive A)
  or 3 (drive C).