The following subsystems comprise the Tcl core:

• Basic utilities (dynamic strings, hashtables, memory allocator)
• Value system (Tcl_Obj, etc.)
• Multi-precision integer support library
• Interpreter core (including procedures, variables and traces)
• Bytecode engine
• Namespaces
• Event handler
• I/O and encoding engine
• Virtual Filesystem engine
• Regular Expression engine
• Package manager
• The clock command
• Child interpreter manager
• Thread support

Note that there are other minor subsystems too (e.g. the format and scan commands), and that the full list of maintenance areas[1] may divide some subsystems into several pieces for ease of maintenance. -- DKF

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Various pages dealing with Tcl internals topics include:

• History of TCL_EVAL_INVOKE

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