Operating system is the collection of services needed to safely interface the hardware with applications. Core topics focus on the mechanisms and policies needed to virtualize computation, memory, and I/O. Overarching themes that are reused at many levels in computer systems are well illustrated in operating Systems (e.g. polling vs interrupts, caching, flexibility costs overhead, similar scheduling approaches to processes, page replacement, etc.). OS should focus on how those concepts apply in other areas of CS - trust boundaries, concurrency, persistence, safe extensibility.