What are two modes of OS? Discuss different OS structures briefly.

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A processor in a computer running Windows has two different modes: user mode and kernel mode.

The processor switches between the two modes depending on what type of code is running on the processor. Applications run in user mode, and core operating system components run in kernel mode. While many drivers run in kernel mode, some drivers may run in user mode.

Simple Structure

It is the simplest Operating System Structure and is not well defined; It can only be used for small and limited systems. In this structure, the interfaces and levels of functionality are well separated; hence programs can access I/O routines which can cause unauthorized access to I/O routines.

Monolithic Structure

The Monolithic operating System in which the kernel acts as a manager by managing all things like file management, memory management, device management, and operational processes of the Operating System.

The kernel is the heart of a computer operating system (OS). Kernel  delivers basic services to all other elements of the System. It serves as the primary interface between the Operating System and the hardware.

In monolithic systems, kernels can directly access all the resources of the operating System like physical hardwareexp KeyboardMouse etc.

Layered Approach

In this type of structure, OS is divided into layers or levels. The hardware is on the bottom layer (layer 0), while the user interface is on the top layer (layer N). These layers are arranged in a hierarchical way in which the top-level layers use the functionalities of their lower-level levels.

In this approach, functionalities of each layer are isolated, and abstraction is also available. In layered structure, debugging is easier as it is a hierarchical model, so all lower-level layered is debugged, and then the upper layer is checked. So all the lower layers are already checked, and the current layer is to be checked only.


Micro-Kernel structure designs the Operating System by removing all non-essential components of the kernel. These non-essential components of kernels are implemented as systems and user programs. Hence these implemented systems are called as Micro-Kernels.

Each Micro-Kernel is made independently and is isolated from other Micro-Kernels. So this makes the system more secure and reliable. If any Micro-Kernel fails, then the remaining operating System remains untouched and works fine.

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