Compatibility: 1.0+
Expects: AH    0eH
DL    drive number (0=A, 1=B, etc.) to become the default disk
                                                                  
Returns: AL    total number of system drives
                                                                  
Info: The drive identified in DL becomes the current DOS default drive
(if it is a valid drive number).

Error test; use DOS Fn 19H (Get CurDisk) to make sure it worked.

Notes:   The return value in AL is number of drives of any type,
including hard disks and 'logical' and phantom drives (such as
drive B: in a 1-floppy system).

  The return value is simply a count of valid drive IDs--not an
indication of which drive IDs are valid.

  This call triggers a critical error (INT 24H) when you select a
drive that does not contain a diskette.

Versions: DOS 3.0+ AL is the same as the value of LASTDRIVE=, as specified
in CONFIG.SYS.  The a typical default LASTDRIVE is 5.

See Also: 3bH (set default directory
INT 11H (equipment check)
INT 24H (critical error handler)
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DOS Fn 0eH: Select DOS Default Disk