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, follow these instructions.

From the Windows Control Panel, find "System & Security or Maintenance".

Click "System", then go into "Advanced System Settings".

Under "Performance", click the "Settings" button.

Select the "Data Execution Prevention" tab.

Select "Turn on DEP for essential Windows Programs and service only" ...OR... Select ""Turn on DEP for all programs and services except those I select". Then, Add "ExtendSim.exe" to the list.