Cc65

cc65 is a cross development package for 6502 and 65C02 targets, including a macro assembler, a C cross compiler, linker, librarian and several other tools.
Overview.
cc65 is based on a native C compiler that was originally adapted for the Atari 8-bit computers by John R. Dunning in 1989, which originated as a Small C descendant. It has several extensions, and some of the limits of the original Small C compiler are gone.
The toolkit has largely been expanded by Ullrich von Bassewitz and other contributors. The actual cc65 compiler, a complete set of binary tools (assembler, linker, etc.) and runtime library are under a license identical to zlib's. The "ca65" cross-assembler is one of the most powerful 6502 cross-assemblers available under an open-source license.
The compiler itself comes close to ANSI C compatibility, while C library features depend on the target platform's hardware. stdio is supported on many platforms, as is Borland-style screen handling. GEOS is also supported on the Commodore 64 and even the Apple II. The library supports many of the Commodore platforms (C64, C128, C16/116/Plus/4, P500 and 600/700 family), Apple II family, Atari 8-bit family, Oric Atmos, Nintendo Entertainment System, Watara Supervision game console and Ohio Scientific Challenger 1P.
Officially supported host systems include Linux, Microsoft Windows, MS-DOS and OS/2, but the source code itself has been reported to work almost unmodified on many platforms beside these.
The ca65 macro assembler supports 6502, 65C02, and 65C816 processors, and can be used standalone without the C compiler.
Supported API.
dynamic.
Note: For static libraries, "Yes" means the feature is available. For dynamic libraries, the columns list the number of available drivers.