Very exciting news! Dave Dunfield has decided to release the source code of over 40 years' work "in the hopes that others may find it useful or maybe learn a few things." This includes the Micro-C compiler, a very nice C compiler for DOS.
You can find everything on Dave's website.
https://dunfield.themindfactory.com/dnldsrc.htm
The source code license isn't an "OSI open source" license, but aims to release the source code for others to use. Thanks Dave!