Types of Kernels

In the early days of Linux, kernels were a single block of code containing all the instructions for the processor, the motherboard, and the other hardware. If you changed hardware, you were required to recompile the kernel code to include what you needed and discard what you did not. Including extra, unneeded code carried a penalty since the kernel became larger and occupied more memory. On older systems that had only 4MB8MB of memory, wasting precious memory for unnecessary code was considered unacceptable. Kernel compiling was something of a "black art" as early Linux users attempted to wring the most performance from their computers. These kernels compiled as a single block of code are called monolithic kernels.

As the kernel code grew larger and the number of devices that could be added to a computer increased, the requirement to recompile became onerous. A new method of building the kernel was developed to make the task of compiling easier. The part of the kernel's source code that composed the code for the device drivers could be optionally compiled as a module that could be loaded and unloaded into the kernel as required. This is known as the modular approach to building the kernel. Now, all the kernel code could be compiled at oncewith most of the code compiled into these modules. Only the required modules would be loaded; the kernel could be kept smaller, and adding hardware was much simpler.

The typical Ubuntu kernel has some drivers compiled as part of the kernel itself (called in-line drivers) and others compiled as modules. Only device drivers compiled in-line are available to the kernel during the boot process; modular drivers are only available after the system has been booted.

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