What is the difference between user and kernel modes in OS?

User mode and kernel mode are two different modes of operation in an operating system, with different levels of access to system resources.

In an operating system, the user mode and kernel mode are two distinct modes of operation. They are used to protect the system from errors and malicious activities, and to ensure the smooth running of the system.

User mode is the less privileged mode, where most applications run. In this mode, the executing code has no ability to directly access hardware or reference memory. The code running in user mode must delegate to system APIs to access hardware or memory. This is a safer mode for running most software, as it prevents user applications from accidentally damaging the system or other applications.

Kernel mode, on the other hand, is the most privileged mode of the operating system. It has unrestricted access to the system's hardware and the entire physical memory. The code running in kernel mode can directly access hardware, interrupt system processes, and manage memory, which makes it powerful but also potentially dangerous. If a bug occurs in the kernel mode, it can cause system-wide problems, including crashes.

The operating system switches between these two modes depending on what needs to be executed. For example, when a user application needs to read from a disk, it will make a system call, which will switch the CPU from user mode to kernel mode. The operating system will then perform the disk read on behalf of the application, before switching back to user mode to return the data.

In essence, the distinction between user mode and kernel mode is a fundamental aspect of operating system design. It provides a way to protect the system's critical functions from being disrupted or misused by user applications, while still allowing those applications to perform necessary tasks.