Η Microsoft announced today that GW-BASIC is now open source. In 1983, Microsoft released GW-BASIC for the IBM PC XT and compatibles. GW-BASIC is a BASIC interpreter derived from IBM's 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this code has been the source language of the 10 assembly language since 1983 February 8088 and must be open for historical reference and educational purposes.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 conwork with the Computer History Museum. In 2018, Microsoft reopened MS-DOS to GitHub. The news archives they contain the original source code and compiled binaries files for MS-DOS 1,25 and MS-DOS 2.0.