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R8C Family General RTOS Concepts

2.6.1 Kernel

An operating system generally consists of two parts: kernel space (kernel mode) and user space (user mode). Kernel is the smallest and central component of an operating system. Its services include managing memory and devices and also to provide an interface for software applications to use the resources. Additional services such as managing protection of programs and multitasking may be included depending on architecture of operating system. There are three broad categories of kernel models available, namely:

Monolithic kernel

It runs all basic system services (i.e. process and memory management, interrupt handling and I/O communication, file system, etc) in kernel space. As such, monolithic kernels provide rich and powerful abstractions of the underlying hardware. Amount of context switches and messaging involved are greatly reduced which makes it run faster than microkernel. Examples are Linux and Windows.

Figure 4 Monolithic Kernel Based Operating System Microkernel

It runs only basic process communication (messaging) and I/O control. The other system services (file system. networking, etc) reside in user space in the form of daemons/servers. Thus, micro kernels provide a smaller set of simple hardware abstractions. It is more stable than monolithic as the kernel is unaffected even if the servers failed (i.e. File System). Examples are AmigaOS and QNX.

Figure 5 Microkernel Based Operating System

RES05B0008-0100/Rev.1.00

January 2010

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