Question

What are the differences between User Mode and Kernel Mode, why and how do you activate either of them, and what are their use cases?

Answer 1

1. 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.

2. User Mode

   In User mode, the executing code has no ability to directly access
   hardware or reference memory. Code
   running in user mode must delegate to
   system APIs to access hardware or
   memory. Due to the protection afforded
   by this sort of isolation, crashes in
   user mode are always recoverable. Most
   of the code running on your computer
   will execute in user mode.

Read more

Understanding User and Kernel Mode