Compatibility: 1.0+
Expects: AH    19H
                                                                  
Returns: AL    DOS current default drive number (0=A, 1=B, etc.)
                                                                  
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)
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DOS Fn 19H: Query DOS Default Disk