HP 3000 Manuals HP 3000 Manuals
Where Are The Files?
Files are kept on disks (unless they are stored on tape), and it is
logical to assume that computers keep the contents of a single disk file
in one, single location. In fact, computers almost never do that.
What they really do is to keep the contents of a single disk file in one
identity. You control the identity of files by giving them names.
Disk memory
A phonograph disk is the device that comes closest to the behavior of a
computer's memory disk. But the phonograph disk allows you to play back
(retrieve) only what is recorded on it. Magnetic or digital tape of the
sort used to record music does permit both recording (input) and playback
(output).
Still, tape has one serious drawback. In order to hear the music
recorded in the middle of the tape, you must first play the music at the
front of the tape and wait. With a phonograph, you can position the tone
arm of the phonograph to play music from any place on the phonograph
record.
Computer memory disks combine the best of both: like tapes, disks can
record and retrieve information. Like the phonograph, computer disks
have a "tone arm," called a read-write head, that records and retrieves
information from any location on the disk.
There is one serious complication. At any moment, several people may be
recording or retrieving information. None of them wants to wait his or
her turn while someone else records or retrieves information.
Two solutions are combined in computer disk management:
* The recording and retrieving of information occur at breathtaking
speed.
* Information is recorded in small chunks, called segments, that are
scattered in locations all across the disk.
This sounds like a prescription for chaos. In fact, it is accomplished
in a highly efficient manner. More to the point, you do not need to
worry that the computer will lose one of the segments of any of your
files.
File recording
File segments are written (recorded) in tracks on a disk. A track, like
the groove in a record, is a prescribed path for the read-write head of
the disk to follow.
If three people are recording information at the same time, the computer
writes a segment of information for the first person, a segment for the
next person, and a segment for the next person. Then it repeats the
process beginning again with the first person. It continues in this
fashion until all of the information for all three people is written to
the disk. Along the way, there is no attempt to keep all of the related
segments together. Instead, the computer creates a map for itself to
direct it to all of all the segments it records.
________________________________________________________________________
|While everyone using the computer is recording and retrieving |
|information from the disk, the computer itself is busy reading |
|and writing information for its own purposes, keeping track of |
|everything it needs to know about its own operations and about your |
|work. |
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Pointing to the right pieces
Controlling the location of a file on the disk is the computer's job.
Your part consists of controlling the identity of the files by giving
names to the files you create. For the most part, you will give names to
files while using a particular program that stores information in files.
Using the EDIT/3000 program's command KEEP allows you to name the file
that you save to disk. In a few cases, however, the program itself will
give names to files that it uses for its own purposes.
________________________________________________________________________
|Many programs create temporary files. These serve as holding |
|places--"scratch pads," if you prefer--while work is in progress. |
|When the work is completed, the program that created the temporary |
|file writes the final version of the work in a permanent file and |
|erases the temporary file. |
________________________________________________________________________
The computer keeps a record of the names (and creators) of all files
created. This record is kept in still another, special file that the
computer itself creates and names.
Along with the name of each file, the computer records the location on
the disk where the first segment of every file is to be found. While you
are recording information in files, while the computer is laying down
segments of files, the computer keeps careful records of the location of
each segment of every file.
The naming of files
Every computer system has rules about the names that you give to files.
Within those rules, you are free to have user names, account names, group
names, and passwords of almost any sort. In MPE/iX, the rules are these:
* Names must consist of at least one character.
* The first character must be a letter of the alphabet.
* The entire name must be no more than eight characters long.
* After the first character, the rest of the characters may be
letters or numbers or a combination of them.
Unless it is your job to set up the computer, you will probably find that
whoever did set up the computer has already established account names,
group names, and user names by the time you start working with the
computer.