If ExtendSim fails on launch with Unknown message or fails when placing a block in a new model with the Unknown block message, it is probably due to a Windows security feature called Data Execution Prevention (DEP).

DEP can help prevent damage to your computer from viruses and other security threats. It does this by stopping the execution of potentially malicious code from the DATA segments of memory.

ExtendSim has an integrated compiler, so it creates and executes compiled code from the DATA segments of memory. DEP can interpret this as malicious data, rather than legitimate code, and stop ExtendSim from properly launching or running.

To access DEP:

Go to the System & Security or Maintenance in Windows Settings.

Click System, then go to Advanced System Settings.

Under Performance, click the Settings button and select the Data Execution Prevention tab.

Since many legitimate applications execute code in the DATA segments of memory, Windows has two methods for allowing applications to be exempt from DEP restrictions:

The default setting for DEP is "Turn on DEP for essential Windows Programs only." If DEP is in this mode, ExtendSim will run properly.

The higher security setting for DEP is "Turn on DEP for all programs and services except those I select". If DEP is in this mode, it will interpret ExtendSim as being a potential threat and prevent ExtendSim from properly running unless ExtendSim is added to the list of exempted programs. To add ExtendSim to the list of programs that allow Data Execution, find the DEP dialog and add ExtendSim.