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