Commodore PET - READING A DATA FILE

When the data file is closed. the PET writes a special mark. called an End of
File
(EOF) mark. on the data tape following the last data item. This mark signais
that the end of the data file has been reached. Later, when the PET is reading a
file, it
will read no further when it encounters an EOF mark, knowing that it has
reached the end of the file.
An EOF mark may be searched for with the STATUS
WORD command
(ST), which checks the file status. ST equals 64 when an EOF
mark has been found. The status word will be usefullater when reading back data
tapes. (For further discussion of the status word, refer to Chapter
4, "Basic
System Functions.")
Mounting the Data Tape
Mount the data file cassette into the tape drive after you have loaded
but before you have executed the program. If you run a program
which is to
read to or write from an external data file (such as a cassette) and there is no
cassette to read to or
write from, the PET will search forever looking for a file on a
cassette that isn't even there! When the data cassette
is mounted, the tape
must be positioned
so that the data file can be located. Certainly if the
cassette
is mounted but the tape is wound to the end, the PET will not be able to
locate, read, or
write any data file.
When writing to a data tape, the PET begins writing
on the tape exactly
where it is positioned. Therefore, be sure that there is nothing at that position on
the cassette which you don't want to lose if the PET writes over it. Also, be sure
there
is plenty of empty space on the cassette following the initial position, so
that
as the PET continues to write to the tape no further information gets written
over.
When reading from a tape, the PET searches forward for the data file
name you have specified
in your OPEN statement. Your data cassette should
therefore
be positioned at some point prior to the beginning of the data file you
wish to read from. If the
PET searches forward and never finds the data file
specified in the
OPEN statement, a FILE NOT FOUND ERROR message is printed
on the screen.
It is a good practice to put only one data file on each side of a cassette, al-
ways beginning the file at or near the start of the tape. By doing this. you
will al-
ways position your tape at the beginning of the tape for both reading and writing.
Furthermore, when
writing to the tape. you will never need to worry about writing
over any other file.
WRITING A DATA FILE
Creating a data file is an important programming technique because it
expands the PET's capabilities to allow processing of large quantities of
data. This
is the emphasis of this section.
To create a data file, LOAD the program file. To use
an external data file, you
must have three statements
within the program file: an OPEN command, a read
or
write command, and a CLOSE commando
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