Expert Answer

Most modern CPUs provide two modes of execution: kernel mode and user mode.

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.

The CPU can execute every instruction in its instruction set and use every feature of the hardware when executing in kernel mode. However, it can execute only a subset of instructions and use only subset of features when executing in the user mode. Having two modes allows designers to run user programs in user mode and thus deny them access to critical instructions.

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