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DCL:MISC１:Summary:File Structures

1.5.1 Mainframe Systems

The access methods and file structures of mainframe-system computers are treated almost totally independent concepts. The file structures consist of the fixed-length (F), variable length (V), fixed-length and blocked (FB), variable-length and blocked (VB), and variable-length, spanned and blocked (VBS) types. However, these concepts do not exist in FORTRAN, and so must be specified in parts outside the FORTRAN programming (such as in the DD statement).

Of these, the fixed-length file structures (F, FB) are the most simple, are are just successions of data of a given length containing no extra codes. In contrast, the variable-length, spanned and blocked (VBS) type has the most complex structure, and is a true variable-length record method that can store a record of any length. (V and VB are variable-length structures but have upper limits to their record lengths.)

Furthermore, when reading or writing, it is generally more efficient for the computer to perform the operation collectively in units that are as large as possible. Thus, the processing speed for blocked structures are usually faster. 

• SEQUENTIAL File

  Normally, the VBS structure is used for the unformatted SEQUENTIAL file. The VBS structure seems to be most consistent with the FORTRAN standard, and so in many cases, this structure is selected when no specifications have been made, such as with the DD statement. 

  Since formatted files are often outputted to devices which have a specific length, F and FB structures are also used. In this case, the user must be careful not to let the maximum record length written by the program exceed the file record length. 

  Outputting unformatted data to F and FB structured files is forbidden since it is extremely inconsistent with the FORTRAN grammar. However, they may be treated as files that contain control codes. In any case, for a user to specify the block length, he will be required to have knowledge  on the system administrator level, far exceeding that of a FORTRAN programmer, and is troublesome. 

  However, from the opposite perspective, this means that it is possible to tune-up programs to the limit by finding the right combination for the nature of the processing performed by the program and the system characteristics. In mainframe- system computers, there are actually significant differences in the processing speed depending on the selection of parameters such a block length. 

• DIRECT File

  Only fixed-length record files can be used as DIRECT files. Since DIRECT files can be randomly accessed, they cannot be blocked. Thus, the SEQUENTIAL file is much faster in sequential access of records.

  Generally, DIRECT files are not easy to handle in mainframe-system computers, and so are not frequently used.