RPG is a programming language for business applications. Originally an initialism for Report Program Generator, it officially no longer stands for anything. Note that while the primary vendor of RPG has been and remains IBM, the language was available from other mainframe and microcomputer manufacturers, including Unisys. The latest incarnation of RPG is RPG IV (aka ILE RPG) on IBM's System i servers; it inherits the System i Integrated Language Environment’s features such as prototyped functions and procedures, static and dynamic binding, access to C routine libraries, dynamic link libraries, and fully recursive and re-entrant modular code.
An RPG program typically starts off with File Specifications, listing all files being written to, read from or updated, followed by Data Definition Specifications containing program elements such as Data Structures and dimensional arrays, much like a "Working-Storage" section of a COBOL program or var statements in a Pascal program. This is followed by Calculation Specifications, which contain the actual meat of the code. Output Specifications can follow which can be used to determine the layout of other files or reports. Alternatively files, some data structures and reports can be defined externally, mostly eliminating the need to hand code input and output specifications.
In the early days of RPG, its major strength was known as the program cycle: every RPG program executes within an implied loop, which can apply the program to every record of a file. At that time each record (individual punch card) would be compared to each line in the program, which would act upon the record, or not, based upon whether that line had an "indicator" turned "on" or "off" — from a set of logical variables numbered 01–99 for user-defined purposes, or other smaller sets based upon record, field, or report processing functions.
Alternatively, the cycle can make an interactive program continue to run until explicitly stopped.
Today, most RPG programmers avoid using the cycle in favor of controlling the flow of the program with standard looping constructs. The concept of level breaks and matching records is unique to the RPG II language. It was originally developed with card readers in mind. RPG III adds some interesting constructs, but the original RPG language is difficult to beat assuming the developer embraces all of the available constructs and features.
FARGO (Fourteen-o-one Automatic Report Generation Operation) was the predecessor to RPG. Both FARGO and RPG were intended to facilitate ease of transition for IBM tabulating machine unit record equipment technicians to the new IBM 1401 series of computers.
Tab machine technicians were accustomed to plugging wires into control panels or plug-boards to implement input, output, control and counter operations (add, subtract, multiply, divide). Tab machines programs were executed by impulses emitted in a machine cycle; hence, FARGO and RPG emulated the notion of the machine cycle with the program cycle. RPG was superior to and rapidly replaced FARGO as the report generator program of choice.
The alternative languages generally available at the time were either COBOL or FORTRAN: COBOL was a verbose business oriented language, FORTRAN was a language that facilitated mathematical applications. Other languages of the era included PL/1, ALGOL, and Autocoder. COBOL was more common in mainframe business operations (System/360 models 30 and above) and RPG was more common in shops that were transitioning from tabulating equipment (System/360 model 20).
RPG III was created for the System/38 and its successor the AS/400 (a mid-range machine). RPG III significantly departed from the original language, providing modern structured constructs like IF-ENDIF blocks, DO loops, and subroutines. RPG III was also available for larger systems including the IBM System/370 mainframe running OS/VS1. It was also available from Unisys for the VS/9 operating system running on the Univac 90/60 mainframe.
DE/RPG or Data Entry RPG was exclusively available on the IBM 5280 series of data-entry workstations in the early 80s. It was similar to RPG III but lacking external Data Descriptions (DDS) to describe data(files) like on the System/38 and its successors. Instead, the DDS part had to be included into the RPG source itself.
RPG/400 with a much cleaner syntax, and tighter integration with the integrated database. This language became the mainstay of development on the AS/400, and its editor was a simple line editor with prompt templates for each specification (type of instruction).
RPG IV (aka RPGLE, aka RPG/ILE) was released in 1994 and the name, officially, was no longer an initialism. RPG IV offered a greater variety of expressions within its new Extended Factor-2 Calculation Specification.
Today, RPG IV is a considerably more robust language. Editing can still be done via the simple editor or it can be edited via PC using IBM's Websphere Development Studio (a customized implementation of Eclipse). IBM is continually extending its capabilities and adding more built-in functions (BIFs). It has the ability to link to Java objects (See IBM's RPG Reference Manual ), and i5/OS APIs; it can be used to write CGI programs with the help of IBM's Cgidev2 web toolkit, RPG xTools CGILIB and other commercial Web enabled packages. Even with the changes it retains a great deal of backward compatibility, so an RPG program written 37 years ago could run today with little or no modification.
OS/400 was later renamed i5/OS to correspond with the new IBM System i5 branding initiative; the 5 was later dropped in favor of just System i. In March 2008 i5/OS was renamed IBM i as part of the Power Systems consolidation of System i and System p product lines. The new Power Systems also adopt more mainstream version numbers, substituting 6.1 for the twenty year old V1R1M0 notation. The latest release is now referred to as IBM i 6.1 and fully supports the RPG IV language, as well as many others. WebSphere Development Studio Client (WDSC) is now referred to as Rational Developer for i (RDi), of which three product levels are available. They are called Rational Developer for i (RDi), RDi Service Oriented Architecture (RDi SOA), and Rational Application Developer (RAD). The new lineup provides in more granular packaging all of the development tools and support previously offered by WDSC and WDSC Advanced Edition.
* Historically RPG is columnar in nature, though free-formatting* is allowed under particular circumstances.* The purpose of various lines code are determined by a* letter code in column 6.* An asterisk (*) in column 7 denotes a comment line* "F" (file) specs define files and other i/o devicesF ARMstF1 UF E K Disk Rename(ARMST:RARMST)* "D" specs are used to define variablesD pCusNo S 6p 0D pName S 30aD pAddr1 S 30aD pAddr2 S 30aD pCity S 25aD pState S 2aD pZip S 10a* "C" (calculation) specs are used for executable statements* Parameters are defined using plist and parm opcodesC *entry plistC parm pCusNoC parm pNameC parm pAddr1C parm pAddr2C parm pCityC parm pStateC parm pZip* The "chain" command is used for random access of a keyed fileC pCusNo chain ARMstF1* If a record is found, move fields from the file into parametersC if %foundC eval pName = ARNm01C eval pAddr1 = ARAd01C eval pAddr2 = ARAd02C eval pCity = ARCy01C eval pState = ARSt01C eval pZip = ARZp15C endif
* RPG makes use of switches. One switch "LR" originally stood for "last record"* LR actually flags the program and its dataspace as removable from memory.C eval *InLR = *On
The same program using free calculations:
* "F" (file) specs define files and other i/o devicesFARMstF1 UF E K Disk Rename(ARMST:RARMST)* "D" specs are used to define variables and parameters* The "prototype" for the program is in a separate file* allowing other programs to call it/copy cust_pr* The "procedure interface" describes the *ENTRY parametersD getCustInf PID pCusNo 6p 0 constD pName 30aD pAddr1 30aD pAddr2 30aD pCity 25aD pState 2aD pZip 10a/free// The "chain" command is used for random access of a keyed filechain pCusNo ARMstF1;// If a record is found, move fields from the file into parametersif %found;pName = ARNm01;pAddr1 = ARAd01;pAddr2 = ARAd02;pCity = ARCy01;pState = ARSt01;pZip = ARZp15;endif;
// RPG makes use of switches. One switch "LR" originally stood for "last record"// LR actually flags the program and its dataspace as removable from memory.*InLR = *On;/end-free
RPG II applications are still supported under the HP MPE operating system on HP3000 and the OpenVMS operating system on VAX, Alpha, Unisys MCP, and Integrity processors by the third party Migration RPG compiler.
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