Date: January 1, 2025

Topic: OS Structure (Monolithic, DOS, Microkernel)

Recall

OS structure is needed to help achieve goals like protection and performance of the OS and services

Notes

OS Structure

The way the OS software is organized with respect to the applications that it serves and the underlying hardware that it manages

Some system services: IPC, File System, Network Access, Mem. Management etc

Importance of OS Structure

Needed for protection, performance, flexibility, scalability, agility and responsiveness

Importantly, protection, performance and extensibility

Goals of OS Structure

Protection: Within and across users and OS itself
Performance: Time taken to perform the services
Flexibility: Extensibility → Not one size fits all, service should adapt to application requirements
Scalability: Performance should improve if hardware resources increase
Agility: Adapting to changes in application needs and/or resource availability
Responsiveness: How quickly OS reacts to external events (important for interactive applications)

However some goals may conflict with each other (e.g., protection vs. flexibility)

Three distinct layers exist, application layer, OS and hardware layer.

Each app in the app has its own address space, protecting them from each other and the OS also has its own address space, protecting the OS from the apps.

Monolithic OSes do not allow for much customization since the system services are already part of the OS.

Monolithic Structure

Application Layer

Applications at top has its own hardware address space

Every application is protected from one-another
Hardware ensures the address space occupied by 1 app is different from another app
This serves as the first level of protection between applications themselves

OS Services and Device Drivers

All services that applications expect from apps are contained in the middle section

OS has its own hardware address space
Includes file systems, network access, scheduling, access to other peripheral devices, etc
As OS and apps have own address space, the OS is protected from the apps
If apps need any service, we switch from hardware address space in the app to the hardware address space of the OS
Monolithic as all services are contained within this blob

Hardware Layer

Hardware at the bottom is managed by the OS (e.g., CPU, memory, peripheral devices)

Performance Issues

While a monolithic structure reduces performance loss by consolidation, it does not allow for customization of OS service for different apps
Customization is needed to fit different app requirements

DOS allows greater customization of OS services by reducing the protection between the apps and the OS. This helps DOS systems achieve greater performance.

Customization can be done on page-replacement, scheduling, interrupts, etc

<aside> 📌 SUMMARY: The three main OS structures (Monolithic, DOS-Like, and Microkernel) each offer distinct trade-offs between protection, performance, and extensibility. Monolithic kernels excel in protection and performance but lack extensibility, DOS-Like systems offer performance and extensibility but sacrifice protection, while Microkernel-based systems provide strong protection and extensibility at the cost of performance.

</aside>