Because ExtendSim has its own compiler, it executes compiled code. Windows can interpret this code as data, rather than code, that is executing. The default setting for Data Execution Prevention (DEP) is Turn on DEP for essential Windows Programs only. Some Windows installations don't use the default; this causes a problem for ExtendSim. In certain cases, ExtendSim will crash upon launch and return the error message Unknown msg Unknown block. To correct this problem, see the FAQ Specific issues to be aware of • Data Execution Prevention (DEP) under the Issues when Installing, Launching, or Opening Files category.