RetroForth is a small, open source implementation of Forth. The latest version was inspired by cmForth, colorForth, eForth, and Pygmy. It also uses some of Chuck Moore's newer ideas.

The source code is clean and elegant. It has a small core, with most of the functionality being provided by optional modules.

Since 2008, RetroForth targets a portable virtual machine called Ngaro. Prior to this it was written specifically for 32-bit x86 systems.

Quick Links:

Major Features:

  • Small: standard image is under 36k bytes (under 9k 32-bit memory cells)
  • Small core written using a custom machine-forth cross-compiler
  • Growing library of extensions providing new features and partial compatibility with other dialects of Forth
  • Direct threaded (dtc) with select inlining of Ngaro machine code
  • Actively developed and supported
  • Simple, understandable source code
  • It's released under a liberal license
  • Support via mailing list, forums, and IRC
  • VM implementations in x86 Assembly, C, C#, F#, ANS FORTH, Go, Java, JavaScript, Lua, Lisp, Perl, PHP, Python, RetroForth, Ruby, and Scheme

MultiPlatformForth tries to classify Forth implementations by the language they were written in. Retro Forth is a bit hard to classify there. From an narrow point of view, it is written in an assembly language – the Ngaro VM, which has 30 instructions. From a wider point of view, since the Ngaro VM has several implementations including a C implementation and a JavaScript implementation ( ), one could say that Retro Forth is written in C or one of those other implementation languages.

Prior versions of RetroForth have served as a base or inspiration for several other Forth systems: