Crosscompiler/Crossassembler for 6502 systemshttp://cc65.github.io/cc65/
A complete cross development package for 65(C)02 systems, including a powerful
macro assembler, a C compiler, linker, librarian and several other tools.
cc65 has C and runtime library support for many of the old 6502 machines,
including the following Commodore machines: VIC20, C16/C116 and Plus/4, C64,
C128, CBM 510 (aka P500), the 600/700 family, newer PET machines (not 2001);
the Apple ][+ and successors; the Atari 8 bit machines; GEOS for the C64, the
C128 and the Apple IIe; the Nintendo Entertainment System (NES); the
Supervision console; the Oric Atmos; the Lynx console.
The libraries are fairly portable, so creating a version for other 6502s
shouldn't be too much work.