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Windows® Internals, Sixth Edition, Part 1 by David A. Solomon Mark E. Russinovich and Alex Ionescu

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Operating System Model

In most multiuser operating systems, applications are separated from the operating system itself—the operating system kernel code runs in a privileged processor mode (referred to as kernel mode in this book), with access to system data and to the hardware; application code runs in a nonprivileged processor mode (called user mode), with a limited set of interfaces available, limited access to system data, and no direct access to hardware. When a user-mode program calls a system service, the processor executes a special instruction that switches the calling thread to kernel mode. When the system service completes, the operating system switches the thread context back to user mode and allows the caller to continue.

Windows is ...

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