What is RPG

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 (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.

Overview
RPG is the mainstay programming language of the IBM System i platform. Originally designed as a query tool, IBM has enhanced the language to become a full-fledged, powerful programming language.
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.

History
RPG is one of the few languages created for punch card machines that is still in common use today. This is because the language has evolved considerably over time. It was originally developed by IBM in the 1960s. The name Report Program Generator was descriptive of the purpose of the language: generation of reports from data files, including matching record and sub-total reports.
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).

Language evolution
RPG II was introduced with the System/3 series of computers. It was later used on System/32, System/34, and System/36, with an improved version of the language. ICL also produced a version on its VME/K operating system.
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 (RPG2 Supported 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.

RPG IV, a modern language
In 2001, with the release of OS/400 V5R1, RPG IV offered even greater freedom for calculations than offered by the Extended Factor-2 Calculation Specification: a free-format text-capable source entry, as an alternative to the original column-dependent source format. The "/FREE" calculation does not require the operation code to be placed in a particular column; the operation code is optional for the EVAL and CALLP operations; and syntax generally more closely resembles that of mainstream, general-purpose programming languages.
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

Platforms
As stated above, the RPG programming language originally was introduced by IBM for their proprietary 1401, /360, /3, /32, /34, /36, /38 AS/400 and System i systems. There have also been implementations for the Digital VAX, Sperry Univac BC/7, Univac system 80, Siemens BS2000, Burroughs B1700, Hewlett Packard HP3000, ICL 2900 series, Honeywell 6220 and WANG VS, as well as miscellaneous compilers & runtime environments for Unix-based systems (INFINITE 36 (formerly UNIBOL36)) and PCs (Baby/400, Lattice-RPG).
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