This document discusses operating system architecture and kernel types. It defines the kernel as the fundamental part of the OS that provides secure access to hardware and decides resource allocation. Kernels can take different forms: monolithic kernels have all services in kernel space for good performance but are difficult to maintain; microkernels minimize the kernel to essential functions and put most services in user space for better modularity but more overhead; hybrid kernels combine aspects of monolithic and microkernels; nano and exokernels are more minimal.

1. Operating System Support
Including
1.Introduction
2.The OS layer
3.Protection
4.Processes & Threads
5.Operating System Architecture

2. Operating System Architecture

3. Definition of Kernel
• The fundamental part of an operating system.
• Responsible for providing secure access to the
machine's hardware for various programs.
• Responsible for deciding when and how long a
program can use a certain hardware.

6. Hardware must provide at least two
execution levels:
• Kernel mode In this mode, the software has
access to all the instructions and every piece
of hardware.
• User mode In this mode, the software is
restricted and cannot execute some
instructions, and is denied access to some
hardware.

7. Types of Kernels
1. Monolithic Kernels
2. Micro kernels
3. Hybrid Kernels
4. NanoKernels
5. Exokernels

9. Monolithic Kernels
• All OS services operate in kernel space
• Good performance
• Execute all of their code in the same address
space (kernel space)
• Rich and powerful hardware access
• Monolithic Kernel : Kernel Image = (Kernel
Core+Kernel Services). When system boots up
entire services are loaded and resides in
memory.
• Example: Windows and Unix.

10. 1. Monolithic Kernels

11. Disadvantages
 Disadvantages
 The dependencies between system components
 A bug in a driver might crash the entire system
 Large kernels → very difficult to maintain
• E.g. Unix, Linux

13. Microkernels
• Minimalist approach
– IPC, virtual memory, thread scheduling
• Put the rest into user space
– Device drivers, networking, file system, user interface
• Run most of their services in user space
→improve maintainability and modularity
• Micro kernel : Kernel Image = Kernel Core. Services are
build into special modules which can be loaded and
unloaded as per need.

14. Microkernel

15. • Disadvantages
– Lot of system calls and context switches
• E.g. Mach, L4, AmigaOS, K42

17. Operating System Architecture
Monolithic Kernel Microkernel
S4
Server: Dynamically loaded Kernel code and data: server program:
17
.......
.......
Key:
S1 .......
S1 S2 S3
S2 S3 S4
Figure 5. Monolithic kernel and microkernel
Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

20. Summary: Kernels
• Monolithic kernels
– Advantages: performance
– Disadvantages: difficult to debug and maintain
• Microkernels
– Advantages: more reliable and secure
– Disadvantages: more overhead
• Hybrid Kernels
– Advantages: benefits of monolithic and microkernels
– Disadvantages: same as monolithic kernels
• Nano kernel & Exo kernels
– Advantages: minimal and simple
– Disadvantages: more work for application developers

21. Thank You