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. 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 was effectively RPG III running on AS/400. IBM renamed the RPG compiler as "RPG/400" but it was identical to the RPG III compiler on System/38. The rebranding took place to differentiate between the earlier CISC (Complete Instruction Set Compile) on the beige boxes to the later, "faster" RISC (Reduced Instruction Set Compile), usually on the black servers. Virtually all IBM products were rebranded as xxx/400 and the RPG compiler was no exception. RPG III compiled with the RPG/400 compiler offered nothing new to the RPG III language until IBM began development of new operation codes, such SCAN, CAT and XLATE after several years of AS/400 availability. These enhancements to RPG III were not available in the System/38 version of RPG III.
RPG IV (a.k.a. RPGLE and ILE RPG) was released in 1994 and the name, officially, was no longer an initialism. RPG IV offered a greater variety of expressions within its Extended Factor-2 Calculation Specification and, later in life, its free-format Calculation Specifications and Procedure syntax. RPG IV is the only version of RPG supported by IBM in its long line of successors to the AS/400 system (iSeries, etc.).
Read more about this topic: IBM RPG
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