Configuring the FreeBSD Kernel

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:

• 
  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.
  
• 
  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.
  
• 
  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

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