platform security

In Symbian 9 and later, the security boundary is between processes, and as DLLs are run inside a single process, from Symbian 9 ownawards each app has to run in its own process (*.exe).

Symbian 9 also changed to a newer ARM instruction set (contributing to why older apps cannot be binary compatible).

For a good introduction, read this wiki article:

http://developer.symbian.com/wiki/display/pub/Platform+Security+-+a+Technical+Overview

and this book, too, perhaps:

http://developer.symbian.com/main/learning/press/books/sops/index.jsp

And for more details, this book:

http://developer.symbian.com/main/learning/press/books/os_internals/index.jsp