x86 is undoubtedly a very complex beast, so ARM/MIPS are fairer comparisons. Never mind the fact that a Xeon CPU is boggling complex on top of that. I think you

x86 is undoubtedly a very complex beast, so ARM/MIPS are fairer comparisons. Never mind the fact that a Xeon CPU is boggling complex on top of that. I think you are right about how problems are more likely to come from interractions with peripherals, especially the MMU. Most of these things are not architecturally specified, so it's an orthogonal problem to ISA complexity. Bugs in instruction decoders are something of a false flag in my opinion. They are no less important than other bugs, but they are much easier to find as well, because the problem space is more manageable. A little more complexity in an instruction decoder is not, in my opinion, enough justification to exclude otherwise useful instructions or functionality.