S51 is a freeware, opensource simulator developed by Daniel Drotos (mailto:drdani@mazsola.iit.uni-miskolc.hu). The simulator is built as part of the build process. For more information visit Daniel's web site at: http://mazsola.iit.uni-miskolc.hu/ drdani/embedded/s51. It currently supports the core mcs51, the Dallas DS80C390 and the Phillips XA51 family.