The Tcl core is the C implementation of Tcl. It is hosted in a fossil repository at core.tcl.tk , and mirrored at mirror1.tcl.tk
Previously, development code was hosted in a CVS repository at sourceforge.net
There is some information about the Tcl core source code on various pages of the wiki, but so far, there is no page dedictated to explaining the source code. If you feel inspired to move this information to a more applicable page, please do!
The prefix Tclp indicates that there may be multiple platform-specific implementations of the function.