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Guru's Guide to SQL Server Architecture and Internals, The by Ken Henderson

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User Mode vs. Kernel Mode

In order to keep misbehaving application code from destabilizing the system, Windows uses two processor modes: user mode and kernel mode. User application code runs in user mode; OS code and device drivers run in kernel mode. Kernel mode has a higher hardware privilege level than user mode and provides access to all system memory and all CPU instructions. By running at a higher privilege than application software, Windows can keep a misbehaving application from directly destabilizing the system.

The Intel x86 family of processors actually supports four operating modes (also known as rings). These are numbered 0 through 3, and each is isolated from the others by the hardware—a crash in a lower-priority mode will not destabilize ...

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