Microsoft announced today that GW-BASIC is now open source. In 1983, Microsoft released GW-BASIC for IBM PC XT and compatible PCs. GW-BASIC is a BASIC interpreter from IBM Advanced BASIC / BASICA.
You can see the Microsoft GW-BASIC source code on GitHub. The GW-BASIC source code is written in assembly language.
According to Microsoft releases the code of GW-Basic because this code has been the source language of the 10 assembly language since 1983 February 8088 and must be open for historical reference and educational purposes. This means that they will not allow it to be modified and exploited in any way.
In addition to GW-BASIC and back in 2014, the Microsoft had released MS-DOS as part of a collaboration with the Museum of Computer History. In 2018, Microsoft reopened MS-DOS on GitHub. The new files there contain the original source code and the compiled binary files for MS-DOS 1,25 and MS-DOS 2.0.