Operating system/Kernel models

Introduction

The kernel is the name given to the program that does most of the housekeeping tasks for the operating system. The boot loader hands control over to the kernel, which initializes the various devices to a known state and makes the computer ready for general operation.

After preparing the hardware for use, the kernel provides an abstraction layer between the hardware and software running on the system. Through an application programming interface (API), software can request the kernel to perform hardware tasks, as well as access many common functions. Most current kernels also multi-task the hardware, sharing it between the various processes running on the system.

Monolithic Kernel

Design

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Monolithic Kernels function with all of the kernel and device drivers inside one address space running in kernel mode. The principle advantage to this approach is efficiency (especially on x86 architectures or 32-bit computer systems where a task switch is a particularly expensive operation). Unfortunately, monolithic kernels suffer the risk of the entire system crashing due to a bug in a device driver, the reason being it is running in the same area with the kernel and other drivers, so, if something fails every other driver and program will be affected.

Pros

Speed
Simplicity of design

Cons

Potential stability issues
Can become huge - Linux 4.15 has 20 million lines of code and Windows 10 contains over 40 million lines!
Potentially difficult to maintain

Examples

Traditional Unix kernels (includes BSDs and Solaris)
Linux
MS-DOS, Windows 9x
Mac OS versions below 8.6

Microkernel

Design

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A Microkernel tries to run most of its services and device drivers in userspace. This can result in an increase in stability and possibly security on machines with a Memory Management Unit. Unlike monolithic kernels, most of the components run in some kind of container, so if something fails, the module or driver will be restarted.

Pros

Stability
Security
Potentially more responsive (though often not in practice)
Benefits for SMP machines

Cons

Additional context switches are usually required
Slow Inter Process Communication can result in poor performance
Not helpful for threads

Examples

AmigaOS
Amoeba
Mach
Minix
MorphOS
L4
QNX

Hybrid kernel

Design

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A hybrid kernel combines the concepts of both monolithic kernels and microkernels. It is generally implemented by having a monolithic kernel with a more microkernel like design. When properly implemented it is hoped that this will result in the performance benefits of a monolithic kernel, with the stability of a Microkernel

Examples

NT kernel (used in Windows NT, 2000, XP, Vista, 7, 8, and 10)
XNU (Mac OS X and ios' kernel)
DragonFly BSD
BeOS
Plan 9