The CMUCL source tree has subdirectories for each major subsystem:
Holds the CMU CL source-file assembler, and has machine specific subdirectories holding assembly code for that architecture.
The CLX interface to the X11 window system.
The Lisp code for the runtime system and standard CL utilities.
The Python compiler. Has architecture-specific subdirectories which hold backends for different machines. The generic subdirectory holds code that is shared across most backends.
The Hemlock editor.
The C runtime system code and low-level Lisp debugger.
CMUCL version of the PCL implementation of CLOS.
System building command files and source management tools.