|
|
|
The most common medium for storage of a device file is magnetic
tape. This section describes the matters that you should keep in
mind when you work with your magnetic tape files.
Every standard reel of magnetic tape designed for digital
computer use has two reflective markers located on the back side
of the tape (opposite the recording surface). As illustrated in
Figure 7-2 "Magnetic Tape Markers", one of
these marks is located behind the tape
leader at the beginning-of-tape (BOT) position, and the other is
located in front of the tape trailer at the end-of-tape (EOT) position.
These markers are sensed by the tape drive itself and their position
on the tape (left or right side) determines whether they indicate
the BOT or EOT positions:
Figure 7-2 Magnetic Tape Markers
As far as the magnetic tape hardware and software are concerned,
the BOT marker is much more significant than the EOT marker because
BOT signals the start of recorded information; but EOT simply indicates
that the remaining tape supply is running low, and the program writing
the tape should bring the operation to an orderly conclusion. The difference
in treatment of these two physical tape markers is reflected by
the file system intrinsics when the file being read, written, or
controlled is a magnetic tape device file. The following paragraphs
discuss the characteristics of each appropriate intrinsic.
FWRITE. If the magnetic tape is unlabeled (as specified in the
HPFOPEN/FOPEN intrinsic or FILE command) and a user
program attempts to write over or beyond the physical EOT marker, the
FWRITE intrinsic returns an error condition code (CCL).
The actual data is written to the tape, and a call to FCHECK reveals
a file error indicating end-of-tape. All writes
to the tape after the EOT marker has been crossed transfer the data
successfully, but return a CCL condition code until the tape crosses
the EOT marker again in the reverse direction (rewind or backspace).
If the magnetic tape is labeled (as specified in the
HPFOPEN/FOPEN intrinsic or FILE command), a CCL
condition code is not returned
when the tape passes the EOT marker. Attempts to write to the tape
after the EOT marker is encountered cause end-of-volume (EOV) labels
to be written. A message then is printed on the operator's console
requesting another reel of tape to be mounted.
FREAD. A user program can read data
written over an EOT marker and beyond the marker into the tape trailer.
The intrinsic returns no error condition code (CCL or CCG) and does
not initiate a file system error code when the EOT marker is encountered.
DDS tape drives do not permit an FREAD past the marker. With DDS
drives, every FWRITE updates the EOT marker and does not permit a
rewrite of previous data.
FSPACE. A user program can space records
over or beyond the EOT marker without receiving an error condition
code (CCL or CCG) or a file system error. The intrinsic does, however,
return a CCG condition code when a logical file mark is encountered.
If the user program attempts to backspace records over the BOT marker,
the intrinsic returns a CCG condition code and remains positioned
on the BOT marker.
FCONTROL (Write EOF). If a user program
writes a logical end-of-file (EOF) mark on a magnetic tape over the reflective
EOT marker, or in the tape trailer after the marker, hardware status is saved
to return END-OF-TAPE on the next FWRITE. The file mark is
actually written to the tape.
FCONTROL (FORWARD SPACE TO FILE MARK). A
user program that spaces forward to logical tape marks (EOFs) with
the FCONTROL intrinsic cannot detect passing the physical EOT
marker. No special condition code is returned.
FCONTROL (BACKWARD SPACE TO FILE MARK). The EOT reflective marker is not
detected by FCONTROL during backspace file (EOF) operations. If the
intrinsic discovers a BOT marker before it finds a logical EOF,
it returns a condition code of CCE and treats the BOT as if it were
a logical EOF. Subsequent backspace file operations requested when
the file is at BOT are treated as errors and return a CCL condition
code and set a file system error to indicate INVALID OPERATION.
In summary, except for FCONTROL, only those intrinsics that cause the
magnetic tape to write information are capable of sensing the physical EOT
marker. If a program designed to read a magnetic tape needed to detect the EOT
marker, it could be done by using the FCONTROL intrinsic to read the
physical status of the tape drive itself. When the drive passes the EOT marker
and is moving in the forward direction, tape status bit 5 (%2000) is set and
remains on until the drive detects the EOT marker during a rewind or backspace
operation. Under normal circumstances, however, it is not necessary to check
for EOT during read operations. The responsibility for detecting
end-of-tape and concluding tape operations in an orderly manner
belongs to the program that originally created (wrote) the tape.
A program that needed to create a multiple reel tape file would normally write
tape records until the status returned from FWRITE indicated an EOT
condition. Writing could be continued
in a limited manner to reach a logical point at which to break the
file. Then several file marks and a trailing tape label would typically
be added, the tape rewound, another reel mounted, and the data transfer
continued. The program designed to read such a multitape file must
expect to find and check for the EOF and label sequence written by
the tape's creator. Since the logical end of the tape may be somewhat
past the physical EOT marker, the format and conventions used to
create the tape are of more importance than determining the location
of the EOT.
|
Complete PDF![[Magnetic Tape Markers]](img/gfx16.gif)