Type III C lass A Program

DIRECT ACCESS I/O
All direct-access files used or created by FORTRAN programs
have file identi'fiers in the following format:
I Filename I Filetype ,
1------------1------------1
I FILE 1 DAxx ,
I I I
xx is the data set reference number, from 01 through 08,.
If the file is being created by a
filemode is Pl. For input to a
filemode of P is acceptable.
FORTRAN program,
FORTRAN program"
the
any
Direct access refers only to those files which are used with
the FORTRAN lanq,uaq.e DEFINE FILE statement. (Note the
distinction between the CMS library DEFINE subroutine in
SYSLIB. TXTLIB wllich is referenced by a CALL DEFINE
statement, and the FORTRAN language statement DEFINE FILE.)
Files used sequentially are not considered direct-access
files,. even though they reside on disk.
Unlike the sequential data set reference numbers, the
direct-access number does not imply any record length. This
information is supplied by the DEFINE FILE s·tatement within
the FORTRAN program. All files are on the permanent disk.
The same data set reference number may not be used for both
a sequential and a direct-access file in the same program,
nor may a single file be referenced by both methods in the
same program. Different access methods may be used for the
same file by different programs, provided the file
identifiers are changed.
The number of records specified in the DEFINE FILE statement
should be realistic. If a new file is being created, the
specified number of records are blanked out on the permanent
disk before the first record is written.. Specifying an
unnecessarily larqe number of records wastes disk space,.
Although the FIND statement is supported, there is no need
to use it in a time-sharing environment,. I/O overlap is
achieved through sharing of CPU time among the virtual
machines. Use of the FIND statement actually slows down
execution of the FORTRAN program slightly. since two
operations must be carried out instead of one.
An example of direct-access I/O is shown in Figure 35.
FORTRAN Programming 333