Specifications

Program files are files that perform a task. They manipulate the
computer and data files in order to achieve a result. MS-DOS is
a collection of program files, as are the control programs found
in application software.
Data files are files that contain the information (data) used by
program files. A file containing a list of names and addresses
would be a data file.
The two types of files are closely related. Program files usually
create data files. These may be the documents created by a
word processor, the numerical array created by a spreadsheet, or
some other collection of information.
A data file containing a written report can’t do anything by
itself. On the other hand, a word processor is useless without
documents to work on.
Filenames and Extensions
Needless to say, each file on a disk must be uniquely
identifiable. To achieve this, MS-DOS requires that you name
every file you create, as you create it.
In order to be able to manipulate files easily, MS-DOS has
certain rules that you must follow when naming files. As you
will see, these rules leave you a great deal of freedom in your
selection of filenames.
Each filename can be up to eight characters long. To this eight
character name can be added an “extension.” Extensions
always begin with a period ( . ) which is followed by up to three
characters.
The format for a valid MS-DOS filename is:
ABCDEFGH.JKL
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Using MS-DOS With Your Computer