SIMH is a free and open source emulator that allows users to run various systems on different platforms. Developed by Robert M. Supnik, a former DEC engineer, SIMH has been in development since the 1960s. Its main objective is to preserve minicomputer hardware and software that would otherwise be forgotten. However, in May 2022, an issue arose when a contributor unilaterally modified the MIT License of SIMH version 4 on GitHub, making it non-free. As a result, Supnik no longer endorses version 4, and a new group called Open SIMH was formed to continue development with Supnik’s support. SIMH can emulate hardware from various companies, allowing users to run different operating systems from different eras.
https://en.wikipedia.org/wiki/SIMH