DOS Fn 19H: Query DOS Default Disk

                                                         Compatibility: 1.0+ 
 Expects: AH    19H
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 Returns: AL    DOS current default drive number (0=A, 1=B, etc.)
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    Info: Returns the drive number of the current DOS default disk.

   Notes: ■ Add 'A' (65H) to AL to obtain a drive letter suitable for use
            in handle-oriented file operations.

See Also: Fn 0eH (set default disk)
          Directory and File Functions
          DOS Functions
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