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how is MESI "an abstraction"? why is it the only model proffered instead of MESIF/MOESI? and "the top 16-bits are basically ignored" is a funny way to spell "g

• dragontamer 8y ago
  > how is MESI "an abstraction"? why is it the only model proffered instead of MESIF/MOESI? Because no CPU actually implements MESI. All CPUs implement more complicated stuff, like MESIF / MOESI. Instead of going into MESIF (which only Intel CPUs implement) or MOESI (only older AMD CPUs implement), lets just stick with the textbook MESI. Which is "wrong", but its "correct enough" to explain the concept. That's what I mean by an abstraction, no CPU today actually does MESI, its simply a concept to introduce to solidify the student's understanding of cache coherency. Its close enough to reality without getting into the tricky CPU-specific details of the real world. > guess we're just handwaving I mean, you have to set those bits back to 0 before using them as a pointer. But the system will literally never use those top 16-bits for anything. So some highly optimized code stores data in those top 16-bits and then zeros them out before using. IIRC, Lisp machines and various interpreters.