Older ARM CPUs had the exeception page (page 0) that had to be mapped into the user's address space. Fortunately that was solved in more recent ARMs with HIGHVECTOR and now you actually set the base address for the exception vectors.
PowerPC has completely split user/kernel space so that address 0 while in the kernel is a different mapping that user access would use. In fact, page 0 on OEA was where all the exception vectors were. One a different note, on many ports I've made access to a 0xdeadbeef address always generate a fault. This was done by reserving +/- 32KB on either side preventing it from being returned by UVM.
