Scheme 48

Scheme 48 is a lightweight and portable implementation of the Scheme programming language. It's known for its focus on modularity and its virtual machine architecture. Unlike some other Scheme implementations, Scheme 48 uses a bytecode interpreter rather than compiling directly to native machine code, which contributes to its portability.

Scheme 48's development was initiated in the late 1980s at MIT. A primary goal was to create a Scheme system suitable for research and experimentation, particularly in areas like programming language design and implementation.

The system is divided into two main parts: the Runtime System and the Compiler. The Runtime System provides the core functionality of the language, including memory management and input/output operations. The Compiler translates Scheme source code into bytecode, which is then executed by the Runtime System.

Scheme 48 pioneered the concept of "programmable compilers," allowing users to extend and modify the compiler's behavior. This feature enables developers to tailor the language to specific needs and experiment with new language features.

The system is distributed under a liberal license, making it freely available for both academic and commercial use. While not as widely used as some other Scheme implementations like Racket, Scheme 48 continues to be used in certain academic and research contexts due to its flexibility and modular design.