Operating systems generally function in two distinct modes: user mode and kernel mode.

In the context of operating systems, these two primary modes of operation serve to differentiate between tasks performed on behalf of the user and those executed by the system itself.

User Mode

User mode, often referred to as problem mode, is where applications run on behalf of the user. In this mode, the executing code is restricted from directly accessing hardware or referencing memory. Additionally, it is designed to prevent user applications from disrupting the operating system or interfering with other applications. This protective measure is crucial for maintaining the stability and security of the operating system. If an application requires special privileges to perform an operation—such as accessing hardware—it must invoke a system call to transition to kernel mode.

Kernel Mode

In contrast, kernel mode is a privileged state in which the operating system operates. Here, the executing code has complete and unrestricted access to the underlying hardware, allowing it to execute any CPU instruction and access any memory address. Kernel mode is reserved for the most fundamental and trusted functions of the operating system. Crashes occurring in kernel mode can be particularly severe, potentially leading to complete system failures.

Transitioning Between Modes

The transition between user mode and kernel mode is managed through the system call mechanism. When a user application needs to request a service from the operating system—such as reading data from a disk—it issues a system call. This action prompts the processor to switch from user mode to kernel mode. After the requested service is fulfilled, the system transitions back to user mode, restoring the application’s execution environment.

This structured separation of modes is essential for protecting the operating system’s integrity and ensuring that user applications operate safely and efficiently.