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COBOL TUTORIAL
FOR BCA STUDENTS OF M G UNIVERSITY |
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A sequential file is a file whose records can be accessed
in the order of their appearence in the file. This
logic of accessing the records in a sequential manner
is independent of the medium used to store a sequential
file. A magnetic tape file, a card file or a printer
file can only have a sequential organization. On the
other hand, a disk file can have different organizations
such as indexed, relative and sequential organizations.
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The task of file handling is the responsibility of
the system software known as IOCS
(Input-Output control system).
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The record of a card file must consist of 80 characters
and those for a print file consist of 80 or 132 characters,
depends on the model of the printer. On the other
hand the size of a tape or disk file may be chosen
by the programmer. Although any suitable size can
be fixed for the records in a tape or disk file, the
choice is subject to certain limitations imposed by
the computer system and its IOCS. A sequential tape
or disk file can contain either fixed or variable-length
records. In most applications fixed-length records
are used.
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While handling a tape or disk file, normally a single
record is not read or written. Instead, the usual
practice is to group a number of consecutive records
to form what is known as a block or physical record.
The actual handling of blocks is transparent to the
programmer. The IOCS takes care of the blocking. The
programmer should only specify the number of records
that should be contained in a block. When a file is
blocked, a physical read or write operation on the
file is only applicable to the entire block and not
to the individual records in the block.
The IOCS reserves a memory space equal to the size
of a block of the file. This memory space is known
as the buffer. The records as defined in the program
are sometimes called logical records and the blocks
which are records as stored on the file medium are
called physical records.
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Modern computers are capable of handling I-O operations
independent of the CPU by means of the hardware known
as data channel. This enable the overlapping of I-O
operations with other CPU operations. To take advantage
of the situation, the IOCS normally requires more
than one buffer for a file. For example, if two buffers
are allocated for an input file, the IOCS can fill-in
one buffer while the program process the records already
read and available in another buffer.
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The blocks constituting a tape file is usually preceded
and followed by records known as the header and trailer
labels respectively. The header label normally contains
information that helps the IOCS to identify the correct
file. The most important of the information stored
in the header label is what is known as the file title.
Normally, tape files should be created with standard
header and trailer labels.
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