The OCD debugger makes use of the memory mapped OCDR register to access the internals of the Atmel AVR target device while it is in stopped mode. When in run mode, the application running on the target can write a value to this register. The OCD system will then signal this to the debugger, which then fetches the data and passes it to the GUI front-end, where it is displayed. The application can thus give primitive debug messages to the debugger.