Operating Systems Structures

The operating system can be implemented with the help of various structures. The structure of the OS depends mainly on how the various standard components of the operating system are interconnected and merge into the kernel. This article discusses a variety of operating system implementation structures and explains how and why they function.

What is a System Structure for an Operating System?

A system structure for an operating system is like the blueprint of how an OS is organized and how its different parts interact with each other. Because operating systems have complex structures, we want a structure that is easy to understand so that we can adapt an operating system to meet our specific needs. Similar to how we break down larger problems into smaller, more manageable subproblems, building an operating system in pieces is simpler. The operating system is a component of every segment. The strategy for integrating different operating system components within the kernel can be thought of as an operating system structure.

Types of Operating Systems Structures

Depending on this, we have the following structures in the operating system:

• Simple Structure
• Layered Structure
• Modular Structure
• Virtual Machines
• Monolithic Structure
• Micro-Kernel Structure
• Hybrid-Kernel Structure
• Exo-Kernel Structure

Simple Structure

Simple structure operating systems do not have well-defined structures and are small, simple and limited. The interfaces and levels of functionality are not well separated. MS-DOS is an example of such an operating system. In MS-DOS, application programs are able to access the basic I/O routines. These types of operating systems cause the entire system to crash if one of the user programs fails. 

Advantages of Simple Structure

• Better Performance: Due to minimal abstraction and fewer interfaces between hardware and software, the OS tends to be faster.
• Ease of Development: Kernel development is simpler as everything is tightly integrated. Developers can directly interact with hardware and internal functions without needing to coordinate across modules.
• Low Overhead: Simple structures avoid the overhead caused by inter-module communication. Memory usage is optimized due to minimal modularization.
• Faster Debugging (in some cases): With all parts tightly coupled, developers can trace bugs more directly—though this can be a double-edged sword.
• Ideal for Small Systems: Suitable for embedded systems or small-scale environments where minimal functionality is required.

Disadvantages of Simple Structure

• Lack of Modularity: No clear separation between functionalities (like file system, memory management, etc.). A change in one part may affect others, making updates and maintenance risky.
• Poor Maintainability: As the OS grows, maintaining the code becomes increasingly difficult. It is harder to isolate faults or enhance specific components.
• No Data Hiding: All parts of the kernel can access each other’s data, violating principles of encapsulation. This increases the chance of bugs and unintended side-effects.
• Difficult to Scale: Cannot be easily extended or adapted for more complex hardware or software requirements.
• Security Risks: With no strict boundaries or protections, bugs or vulnerabilities in one component may compromise the entire system.

Layered Structure

An OS can be broken into pieces and retain much more control over the system. In Layered structure, the OS is broken into a number of layers (levels). The bottom layer (layer 0) is the hardware and the topmost layer (layer N) is the user interface. These layers are so designed that each layer uses the functions of the lower-level layers. This simplifies the debugging process, if lower-level layers are debugged and an error occurs during debugging, then the error must be on that layer only, as the lower-level layers have already been debugged. The main disadvantage of this structure is that at each layer, the data needs to be modified and passed on which adds overhead to the system. Moreover, careful planning of the layers is necessary, as a layer can use only lower-level layers. UNIX is an example of this structure. 

Advantages of Layered Structure

• Modularity: Each layer is designed to perform a specific function, promoting clean separation of concerns. This makes the system easier to understand, develop and test.
• Ease of Enhancement: Individual layers can be updated or replaced without affecting the rest of the system. Facilitates evolution and scalability of the OS.
• Simplified Debugging and Testing: Since the structure is modular, errors can be isolated to specific layers. Testing can be done layer by layer, making fault localization easier.
• Improved Maintainability: The modular nature allows for simpler maintenance and upgrades. Developers can work on specific components without needing full knowledge of the entire system.
• Data Hiding and Encapsulation: Each layer hides its implementation details from the higher layers. Promotes abstraction, enhancing system security and stability.
• Better Organization of Code: Code is logically organized, reducing complexity and duplication.

Disadvantages of Layered Structure

• Performance Overhead: The layered approach may lead to extra processing and delays, as each system call must pass through multiple layers. This reduces the speed compared to a monolithic/simple structure.
• Rigid Structure: Strict layer boundaries can sometimes make communication between non-adjacent layers difficult or inefficient. A function in the top layer must rely solely on the lower layer interfaces.
• Complex Design Requirements: Designing a layered OS requires careful planning to ensure proper functionality between layers.
• Dependency Management: Managing dependencies between layers becomes critical. A bug or limitation in a lower layer can propagate upward.
• Increased Code Duplication (in some cases): If layers don’t share utilities efficiently, similar operations might be duplicated across layers, increasing codebase size.

Modular Structure

It is considered as the best approach for an OS. It involves designing of a modular kernel. The kernel has only a set of core components and other services are added as dynamically loadable modules to the kernel either during runtime or boot time. It resembles layered structure due to the fact that each kernel has defined and protected interfaces, but it is more flexible than a layered structure as a module can call any other module. For example Solaris OS is organized as shown in the figure.

Virtual Machines (VMs)

Based on our needs, a virtual machine abstracts the hardware of our personal computer, including the CPU, disc drives, RAM and NIC (Network Interface Card), into a variety of different execution contexts, giving us the impression that each execution environment is a different computer. An illustration of it is a virtual box. An operating system enables us to run multiple processes concurrently while making it appear as though each one is using a different processor and virtual memory by using CPU scheduling and virtual memory techniques.

The fundamental issue with the virtual machine technique is disc systems. Let's say the physical machine only has three disc drives, but it needs to host seven virtual machines. The program that creates virtual machines would need a significant amount of disc space in order to provide virtual memory and spooling, so it should be clear that it is impossible to assign a disc drive to every virtual machine. The answer is to make virtual discs available.

Advantages of Virtual Machines (VMs)

1. Hardware Abstraction: VMs simulate real hardware, allowing multiple OS environments to run on a single physical machine. Each VM behaves like an independent system with its own CPU, memory, storage and network interface.
2. Efficient Resource Utilization: Physical resources (CPU, RAM, storage) can be shared dynamically across VMs. Increases hardware usage efficiency and reduces idle time.
3. Isolation and Security: Each VM operates in an isolated environment—issues or crashes in one VM don’t affect others. Enhances system security, especially useful for testing untrusted applications.
4. Flexibility and Portability: VMs are typically stored as files, making it easy to move or copy them across systems. Helps in disaster recovery, testing and development.
5. Support for Legacy Software: Older operating systems or applications can run in VMs, even if they're not compatible with the host OS.
6. Snapshots and Rollbacks: Users can take snapshots of a VM’s current state and restore it later—ideal for testing and training.
7. Simplified Deployment: Preconfigured VM images can be deployed quickly, reducing system setup time.

Disadvantages of Virtual Machines (VMs)

1. Performance Overhead: VMs are slower than native execution because of resource abstraction and virtualization layers. Not ideal for high-performance computing or heavy I/O tasks.
2. Disk and Memory Limitations: Virtualization consumes significant RAM and disk space. Running multiple VMs may exceed physical limitations, especially on storage-intensive systems (as in your disk drive example).
3. Complex Resource Management: Difficult to allocate and manage storage (e.g., only 3 physical disk drives for 7 VMs). Requires techniques like virtual disks, thin provisioning and shared storage systems.
4. Requires Powerful Host System: Hosting several VMs needs a high-end machine with ample CPU cores, RAM and disk space.
5. Security Risks in Misconfigured Systems: Poorly configured hypervisors or guest VMs can create vulnerabilities or VM escape attacks.
6. Limited Access to Physical Hardware: Some hardware-dependent applications (e.g., those needing GPU acceleration or specific peripherals) may not work properly inside VMs.
7. Licensing and Management Overhead: Running multiple OSes might increase software licensing costs and require more administrative effort.

Monolithic Structure

A monolithic structure is a type of operating system architecture where the entire operating system is implemented as a single large process in kernel mode. Essential operating system services, such as process management, memory management, file systems and device drivers, are combined into a single code block

Micro-Kernel Structure

Micro-Kernel structure designs the operating system by removing all non-essential components from the kernel and implementing them as system and user programs. This results in a smaller kernel called the micro-kernel. Advantages of this structure are that all new services need to be added to user space and does not require the kernel to be modified. Thus it is more secure and reliable as if a service fails, then rest of the operating system remains untouched. Mac OS is an example of this type of OS. 

Hybrid-Kernel Structure

Hybrid-Kernel structure is nothing but just a combination of both monolithic-kernel structure and micro-kernel structure. Basically, it combines properties of both monolithic and micro-kernel and make a more advance and helpful approach. It implement speed and design of monolithic and modularity and stability of micro-kernel structure.

Exo-Kernel Structure

Exokernel is an operating system developed at MIT to provide application-level management of hardware resources. By separating resource management from protection, the exokernel architecture aims to enable application-specific customization. Due to its limited operability, exokernel size typically tends to be minimal. The OS will always have an impact on the functionality, performance and scope of the apps that are developed on it because it sits in between the software and the hardware. The exokernel operating system makes an attempt to address this problem by rejecting the notion that an operating system must provide abstractions upon which to base applications. The objective is to limit developers use of abstractions as little as possible while still giving them freedom.