Protected Memory and the Kernel
Since physical memory is only used when allowed by the kernel mode code of the operating system, it is also known as protected memory. In early operating systems, such as MS-DOS, it was simple for a badly coded program to overwrite a section of memory that it should not. The results could be unpredictable, but usually resulted in a system crash. By separating the operating system into user mode and kernel mode, and thus protecting memory with privileged code, user applications will not find it easy to access memory locations that they should not.
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