INT 21h (33)             DOS Serivce Calls

    Most of the DOS functions are implemented as sub-functions of DOS 21h,
    which are listed under Services in the DOS menu.

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       Note:          In general, you should choose to use the DOS service
                      under INT 21h when you have a choice between an
                      interrupt (like INT 27h) and a DOS service (like INT
                      21h, function 31h).

                      The "See also" reference above will take you to the
                      first DOS service, function 00h.