 SCP-603 is a self-replicating computer program capable of reproducing and extending its own source code. On startup, SCP-603 begins generating new versions of its source code in a separate thread of execution. SCP-603 does not modify itself in memory. Instead, it keeps an internal copy of the source code, which is modified iteratively. On a clean termination of SCP-603, the entire source code will be output to the working directory in a new directory, named Source. In addition to modifications to the internal and external functionality of the program, alterations in additions to the semantics and structure of the language are made with each iteration.