IIRC, the IBM/360 exception model is such that if an exception occurs, there instruction in question cannot have any side effects -- no partial execution. That

IIRC, the IBM/360 exception model is such that if an exception occurs, there instruction in question cannot have any side effects -- no partial execution. That leads to a clean exception model but can cause performance pain. Say the instruction is a string copy. The instruction itself might cross a page boundary, and the source might cross a page boundary (perhaps more than one), and the destination can also cross a page boundary. The microcode can't just begin processing the instruction, then hit a page fault, fix up the page mapping, and continue. Instead it must check that each possible read or write will not cause a page fault and only then proceed with the entire sequence.