The PowerPC Book E architecture introduced the first major change to the PowerPC architecture since the original Green Book PowerPC processors were introduced. Central to the Book E architectural changes is the MMU which is always in translation mode, even during exception processing. This presented some unique challenges for cleanly integrating the architecture into the Linux/PPC kernel.
In addition to the base PowerPC Book E architecture changes, the first IBM PPC440 core implementation included 36-bit physical addressing support. Since I/O devices are mapped above the native 32-bit address space, providing support for this feature illuminated several limitations within the kernel resource management and mapping system.