Microsoft announced today that GW-BASIC is now open code. In 1983, Microsoft released GW-BASIC for the IBM PC XT and compatibles. GW-BASIC is a BASIC interpreter derived from the Advanced BASIC / BASICA by IBM.
You can see the Microsoft GW-BASIC source code on GitHub. The source code of GW-BASIC is written in the language assembly.
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this code has been the source language of the 10 assembly language since 1983 February 8088 and must be open for historical reference and educational purposes.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 partnership with the Computer History Museum. In 2018, Microsoft re-opened MS-DOS on GitHub. The news archives they contain the original source code and compiled binaries files for MS-DOS 1,25 and MS-DOS 2.0.