cc65

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.{{Cite web|url=https://github.com/cc65/cc65/commit/aeb849257277a6b98542de8579697b81c6dd70e6|title = Simplified license. · cc65/Cc65@aeb8492|website = GitHub}}

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 {{mono|conio.h}} screen handling. GEOS is also supported on the Commodore 64 and the Apple II. The library supports many of the Commodore platforms (C64, C128, C16/116/Plus/4, P500 and 600/700 family), Apple II, Atari 8-bit computers, Oric Atmos, Nintendo Entertainment System,{{Cite book |last=Hugg |first=Steven |title=Making Games for the NES |publisher=Amazon Digital Services LLC |year=2019 |isbn=9781075952722 |pages=231–232 |language=en}}{{Cite book |last=Cruise |first=Tony |title=Classic Game Programming on the NES |publisher=Manning Publications |year=2024 |isbn=9781633438019 |pages=268 |language=en}} Watara Supervision game console, Synertek Systems SYM-1 and Ohio Scientific Challenger 1P.{{Cite web|url=https://cc65.github.io/doc/osi.html|title = Ohio Scientific-specific information for cc65}}

Officially supported host systems include Linux, Microsoft Windows, MS-DOS and OS/2, but the source code itself has been reported{{by whom|date=November 2017}} 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.[https://cc65.github.io/doc/ca65.html ca65 Users Guide]

• conio (text-based console I/O non-scrolling)
• dio (block-oriented disk I/O bypassing the file system)

• em (expanded memory, used for all kinds of memory beyond the 6502's 64K barrier, similar EMS)
• joystick (relative input devices)
• mouse (absolute input devices)
• serial (communication)
• tgi (2D graphics primitives inspired by BGI)