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THE, Layers Pro: -easy to debug -easy to verify -modularity abstraction Cons: -may nto be efficient (database may not use filesystem (FS cache),it talks to disk directly),you go the kernel,you introduce overhead -require careful design -ned to avoid circular dependencies The first time to design system, it's easy to use layer design, may be the second time you don't need them. -Synchronization Shared Memory Nucleus -small nucleus for supporting multiple simultaneous OS(user level process) implementation -central abstraction is the concurrent process -synchronization via message passing how is is different from Virtual Machine? -VM wants to virtual hardware, different purpose. Build different flavor of OS (batch, multi-media, real-time). Cons and Pros of message passing -clear, hard to mess up -less efficient Hydra: -Capability(reference to an object, attached to a write, e.x: file discriptor has capability with itself, from the user-point) b...