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DDS = Data Description Specifications

This "programming language" is used to define the layout of files, both physical and logical, interactive and prompt screens, reports, and various other kinds of objects in the 400 family.

On S/36 SSP prior to RPG/400 and DDS, we were able to use alternate index files, which were conceptually similar to the logical access files of 400 today.

The objects created by DDS do not DO anything like a program, but rather they are layouts for the data to be processed by a program.

Think similar to SDA which is one way that menus, screens, and so forth were created prior to DDS; and also UIM which BPCS uses to create help screens. Prior to RPG/400 we did not use externally defined print files, but internally defined then within the programs that were generating the reports.

Data Files

Reference Files

Reference files do not contain any data ... they are only used to establish appliation wide field definitions.

A physical file will define a reference file above the first record format specification. In the individual field definitions for the record format, the REFFLD keyword will used to define what field in the reference file the new field will be patterned after.

A                                      REF(REFFILE)                 
A          R FORMAT                    TEXT('Record format')     
A            NEWFIELD  R               REFFLD(REFFIELD)            
A                                      TEXT('Field description')     
A            OTHFIELD  R               REFFLD(REFFIELD1 OTHERREF)     

The REFFLD keyword can also specify a different reference file if the field is not defined in the primary reference file.

Any physical file can be used as a reference file ... but most applications establish a single file to be the reference.

Reference files can be used in any other file that defines new fields (physical files, display files, printer files, etc).

Physical Files

Physical files are represented on the system by *FILE objects with the PF-DTA attribute.

Physical files can be purely sequential or have access paths defined by keys in DDS.

Physical files are created using the CRTPF command.

Source Physical Files

Source physical files are represented on the system by *FILE objects with the PF-SRC attribute.

They are created using the CRTSRCPF command.

Logical Files

Logical files are alternate access paths to related physical files and contain no data directly.

Logical files are represented on the system by *FILE objects with the LF attribute.

They are defined by DDS.

Display Files

Printer Files

Communication Files