memory layout

Hello everybody, I am still working on the port for openrisc and have now some questions concerning the memory layout. (kern/xxx/mem_layout-xxx.cpp) What parts are necessary, and what do they mean? For example in the arm architecture there is Map_x, Caps_x, Kernel_x, Pmem_x... Are there any restrictions on how big/small one area has to be? What is the phys_to_pmem function supposed to do? (what does pmem stand for?) Thanks Tobias