RetroForth is a minimalistic implementation of Forth.
Old versions (1 - 9) were written in x86 assembly (using either NASM or FASM). Depending on the version, it could run on any number of platforms including natively (as a ForthOS), DOS, Windows, Linux, FreeBSD, NetBSD, OpenBSD, BeOS, and SCO OpenServer.
The current version is written in a machine Forth dialect and runs on a highly portable virtual machine called Ngaro.
The primary goal of RetroForth is to be small and understandable, not to be ANS compliant.