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When the target ICE is set to Simulator through [Project] - [Debug], "_default.cs8 file cannot be opened. Check if _default.cs8 file or the folder is not write-protected." error is output.
The administrator privilege may not be set to DTU8. Set the administrator privilege following the procedure below, and re-execute. (1) With the Explorer, go to C:\Program Files\U8dev\Bin folder (for 32-bit version OS) or C:\Program Files (x86)\U8dev\Bin (for 64-bit version OS), and while DTU8.exe is selected, click the right button on the mouse, and select [Properties]. (2) In the displayed [DTU8.exe Properties] dialog, check "Run this program as an administrator" in [Privilege level], and click [OK].
Also, refer to U8 Development Environment Release Note (ReleaseNote_j.pdf) that cautions are pointed out for Windows7 64-bit version. It is copied to your PC along with User's Manual when installing the development environment. Please read the document (Also available in the root folder of "U8 Development Tools" Setup CD).