RPG (Report Program Generator) is a high-level programming language designed for business applications, particularly for generating reports on IBM midrange systems. RPG was originally developed by IBM in the late 1950s and has evolved over the years with various versions and enhancements. It gained popularity as a language for writing programs that produce reports from business data. Here are key aspects of RPG:

1. Report Generation Focus:
   • RPG was initially designed as a language for generating reports. It excels at handling tabular data and producing formatted output for business reports.
2. IBM Midrange Systems:
   • RPG was originally associated with IBM midrange systems, such as the IBM System/3, System/32, System/34, and later the AS/400 (now known as IBM i).
   • It became a standard language for developing applications on these midrange platforms.
3. Column-Based Specifications:
   • RPG uses a column-based specification format, where each column has a specific meaning. Columns are used to define fields, operations, and other elements of the program.
4. Fixed-Format Source Code:
   • RPG traditionally uses fixed-format source code, where each statement begins in a specific column. This format facilitates a straightforward and concise coding style.
5. Cycle-based Execution:
   • RPG programs are often organized into cycles, and the execution of the program progresses through these cycles. The cycle-based model includes specifications for input, processing, and output.
6. Data Description:
   • RPG includes built-in data description capabilities for defining fields, records, and files. It supports alphanumeric, numeric, and date data types.
7. Calculation Specifications:
   • RPG uses calculation specifications for defining business logic. These specifications include operations for arithmetic, conditional logic, and data manipulation.
8. Data-Centric Approach:
   • RPG has a data-centric approach, where data definitions play a central role. Data files and their structures are defined explicitly in the program.
9. Database Interaction:
   • RPG programs can interact with databases on IBM midrange systems. They can perform database operations such as reading, updating, and writing records.
10. RPG II and RPG III:
    • RPG II and RPG III are later versions that introduced improvements and additional features. RPG III, for example, added support for more modern programming constructs and more advanced database capabilities.
11. Integrated Language Environment (ILE RPG):
    • With the evolution of IBM midrange systems, RPG has been enhanced to become part of the Integrated Language Environment (ILE). ILE RPG provides additional features and integration capabilities.
12. Modernization Efforts:
    • While traditional RPG remains in use, efforts have been made to modernize RPG applications. IBM i continues to support RPG, and newer versions offer features for modern development practices.

RPG has been widely used in the IBM midrange environment for decades, and many businesses have built critical applications using RPG. Despite its historical association with report generation, RPG has evolved to support a broader range of application development needs on IBM i systems.