Commodore 1541-II - Chapter 5: Sequential Data Files

CHAPTER 5
SEQUENTIAL DATA FILES
THE CONCEPT OF FILES
A file on a diskette is just like a file cabinet in your office—an organized place to put
things. Nearly everything you put on a diskette goes in one kind of file or another. So far
all we've used are program files, but there are others, as we have mentioned. In this
chapter we will learn about sequential data files.
As we just suggested, the primary purpose of a data file is to store the contents of
program variables, so they won't be lost when the program ends. A sequential data file is
one in which the contents of the variables are stored "in sequence," one right after
another, just as each link in a chain follows the previous link. You may already be familiar
with sequential files from using a DATASSETTE™, because sequential files on diskette
are just like the data files used on cassettes. Whether on cassette or diskette, sequential
files must be read from beginning to end, without skipping around in the middle.
When sequential files are created, information (data) is transferred byte by byte,
through a buffer, onto the magnetic media. Once in the disk drive, program files,
sequential data files, and user files all work sequentially. Even the directory acts like a
sequential file.
To use sequential files properly, we will learn some more Basic words in the next
few pages. Then we'll put them together in a simple but useful program.
Note: Besides sequential data files, two other file types are recorded sequentially
on a diskette, and may be considered varying forms of sequential files. They are
program files, and user files. When you save a program on a diskette, it is saved in
order from beginning to end, just like the information in a sequential data file. The
main difference is in the commands you use to access it. User files are even more
similar to sequential data files—differing, for most purposes, in name only. User
files are almost never used, but like program files, they could be treated as though
they were sequential data files and are accessed with the same commands.
For the advanced user, the similarity of the various file types offers the
possibility of such advanced tricks as reading a program file into the computer a
byte (character) at a time and rewriting it to the diskette in a modified form. The
idea of using one program to write another is powerful, and available on the
Commodore disk drives.
OPENING A FILE
One of the most powerful tools in Commodore Basic is the Open statement. With it,
you may send almost any data almost anywhere, much like a telephone switchboard that
can connect any caller to any destination. As you might expect, a command that can do
this much is fairly complex. You have already used Open statements regularly in some of
your diskette housekeeping commands.
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