In April 1999, newLISP was ported to Linux; some of its core algorithms were rewritten, and all Windows-specific code removed. newLISP was released as an open-source software project licensed under the GPL, and development on Windows stopped after version 6.0.25. During the first half of 2001, newLISP was ported back to Windows on the Cygwin platform without graphics abilities. In the second half of 2001, a cross-platform Tcl/Tk frontend named newLISP-tk was released around version 6.3.0. In 2006, 64-bit precision was introduced for integer arithmetic and for some operations on files in version 9.0.