Llvm-mos
From Foenix F256 / Wildbits/K2 Wiki
The llvm-mos project is a suite of development tools aimed at 65xx processors. It comes with optimizing C and C++ compilers that are mostly compatible with the C99 and C++11 standards, allowing for modern programming styles.
Foenix F256 support, programming libraries, examples, and installers for Windows, MacOS, and Linux can be found at Kangaroo Punch Studios.