S9fES is a mature, portable, and comprehensible interpreter for R4RS Scheme. The core interpreter is written in pure ANSI C (C89), so it runs on virtually any platform offering a C compiler. It can even be compiled natively on Plan 9. On Unix-based platforms (plus Cygwin and MinGW), it also offers
The internals of the interpreter are discussed in detail in the Scheme 9 Book
The code of the interpreter is in the public domain. You can download it right here:
Mirrors:     - Thanks!