IBM 5150 Manual To Operations page 112

Files are used so that DOS can find specific information
easily, so that information that isn't needed isn't taking
up room in the computer too. (For example, you don't
need your bowling team averages at the same time as
you need the form letter.)
You usually have a number of files on a single diskette.
You can have up to 64 files on a diskette. Sometimes
the files on one diskette are related to each other (like
the programs and data files to keep track of a
company's inventory); and sometimes the files have
been put on whatever diskette was handy.
It doesn't matter what combination of files is on a
diskette. What matters is that each file has a unique
name.
That means that every name on a diskette has to be
different-but you can have the same name on two
different diskettes.
For our examples a little earlier, the names of these files
m~~: ~
BOWL
LETTER
BIRTHDAY
BIRTHDAY.BAS
A file's name is made up of a filename and an extension.
In DOS, filenames are from one to eight characters long.
The characters in a filename can be:
• the letters of the alphabet
• the numbers 0 through 9
•
and these special characters­
$ #&@! %() - { }
<>'
\ 1\
~
I '
A filename can be followed by an optional short name
called an extension. An extension starts with a period,
has_one, two, or three characters, and follows immediately
after the filename.
DOS 3-38