cc65 was originally a C compiler for the Atari 8-bit machines written by John R. Dunning. In prior releases I've described the compiler by listing up the changes made by me. I have made many more changes in the meantime (and rewritten major parts of the compiler), so I will no longer do that, since the list would be too large and of no use to anyone. Instead I will describe the compiler in respect to the ANSI/ISO C standard. In fact, I'm planning a complete rewrite (that is, a complete new compiler) for the next release, since there are too many limitations in the current code, and removing these limitations would mean a rewrite of many more parts of the compiler.
There are separate documents named library.html and funcref.html that cover the library that is available for the compiler. If you know C, and are interested in doing actual programming, the library documentation is probably of much more use than this document.
If you need some hints for getting the best code out of the compiler, you may have a look at coding.html which covers some code generation issues.