Updated for FreeBSD 5.X by Joel Dahl.
1、Why Build a Custom Kernel?
Traditionally, FreeBSD has had what is called a “monolithic” kernel. This means that the kernel was one large program, supported a fixed list of devices, and if you wanted to change the kernel's behavior then you had to compile a new kernel, and then reboot your computer with the new kernel.
Today, FreeBSD is rapidly moving to a model where much of the kernel's functionality is contained in modules which can be dynamically loaded and unloaded from the kernel as necessary. This allows the kernel to adapt to new hardware suddenly becoming available (such as PCMCIA cards in a laptop), or for new functionality to be brought into the kernel that was not necessary when the kernel was originally compiled. This is known as a modular kernel.
Despite this, it is still necessary to carry out some static kernel configuration. In some cases this is because the functionality is so tied to the kernel that it can not be made dynamically loadable. In others it may simply be because no one has yet taken the time to write a dynamic loadable kernel module for that functionality.
Building a custom kernel is one of the most important rites of passage nearly every BSD user must endure. This process, while time consuming, will provide many benefits to your FreeBSD system. Unlike the GENERIC kernel, which must support a wide range of hardware, a custom kernel only contains support for your PC's hardware. This has a number of benefits, such as:
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Faster boot time. Since the kernel will only probe the hardware you have on your system, the time it takes your system to boot can decrease dramatically.
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Lower memory usage. A custom kernel often uses less memory than the GENERIC kernel, which is important because the kernel must always be present in real memory. For this reason, a custom kernel is especially useful on a system with a small amount of RAM.
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Additional hardware support. A custom kernel allows you to add in support for devices which are not present in the GENERIC kernel, such as sound cards.
2、Building and Installing a Custom Kernel
First, let us take a quick tour of the kernel build directory. All directories mentioned will be relative to the main /usr/src/sys directory, which is also accessible through the path name /sys. There are a number of subdirectories here representing different parts of the kernel, but the most important for our purposes are arch/conf, where you will edit your custom kernel configuration, and compile, which is the staging area where your kernel will be built. arch represents one of i386, alpha, amd64, ia64, powerpc, sparc64, or pc98 (an alternative development branch of PC hardware, popular in Japan). Everything inside a particular architecture's directory deals with that architecture only; the rest of the code is machine independent code common to all platforms to which FreeBSD could potentially be ported. Notice the logical organization of the directory structure, with each supported device, file system, and option in its own subdirectory. Versions of FreeBSD prior to 5.X support only the i386, alpha and pc98 architectures.
This chapter assumes that you are using the i386 architecture in the examples. If this is not the case for your situation, make appropriate adjustments to the path names for your system's architecture.
Note: If there is not a /usr/src/sys directory on your system, then the kernel source has not been installed. The easiest way to do this is by running sysinstall (/stand/sysinstall in FreeBSD versions older than 5.2) as root, choosing Configure, then Distributions, then src, then sys. If you have an aversion to sysinstall and you have access to an “official” FreeBSD CDROM, then you can also install the source from the command line:# mount /cdrom# mkdir -p /usr/src/sys# ln -s /usr/
