All of 3.9's exec functions will also be in ExecSG.
There is a backwards compatibility mode for compiling source code from 3.9 that will mostly be preprocessor based. That means that some things that work differently on ExecSG (libraries for example) are encapsulated into preprocessor macros so that most source code can be recompiled on 4.0 without too many changes.
Changes mostly affect startup (Libraries need a slightly different opening sequence), and of course some things like Interrupt handling are different.
Also, PCI support is unified under a common API now.
More work will be required for devices and libraries, since the model is significantly different from the old exec. However, it isn't too difficult, I managed to convert the ramdev device in a two or three hours.
