了解用户和内核模式
http://blog.codinghorror.com/understanding-user-and-kernel-mode/
In Kernel mode,the executing code has complete and unrestricted access to the underlying hardware. It can execute any cpu instruction and reference any memory address. Kernel mode is generally reserved for the lowest-level,most trusted functions of the operating system. Crashes in kernel mode are catastrophic; they will halt the entire PC.
