Prefetch Abort handler
If the system has no MMU, the Prefetch Abort handler can report the error and quit. Otherwise the address that caused the abort must be restored into physical memory.
lr_ABT points to the instruction at the address following the one that
caused the abort, so the address to be restored is at
lr_ABT-4. The virtual
memory fault for that address can be dealt with and the instruction fetch retried. The
handler therefore returns to the same instruction rather than the following one, for