以前老听说编译内核,觉得特别神气,呵呵,特别是像FreeBSD这样自由开发的操作系统,简直是非常好,能够自己定制内核,想怎么弄就怎么弄,呵呵,能够提升速度,还能够保证安全稳定,所以我就试一试怎么定制内核. (我也是新手，如果文章出现错误，请指正！)
我所用的FreeBSD是5.2.1版,现在是比较新的了,我一看我的内核文件大小:
# ls -l /boot/kernel/kernel
-r-xr-xr-x 1 root wheel 5940286 Feb 24 2004 /boot/kernel.bak/kernel
看来不小呀，5M多，呵呵，我要把它编成 3M左右,呵呵，begin...
一、安装内核源代码
要编译内核，首先必须保证内核的源代码在，我们看看我们的系统有没有安装：
# ls /usr/src/sys
ls: /usr/src/sys: No such file or directory
证明我们并没有安装内核源代码，好，我们就安装它。内核源代码默认是安装在了/usr/src/sys目录下的，如果不存在该目录，那么证明你没
有安装源代码。
我们先把我们的FreeBSD的安装光盘放进光驱，然后在FreeBSD下执行以root身份执行/stand/sysinstall， 选择Configure，接着是
Distributions， 下来src， 最后sys,回车后能开始安装了，一般只要几分钟就能安装好，装好了你就能查看该文件夹：
# ls /usr/src/sys
Makefile ddb libkern netkey pci
alpha dev modules netnatm posix4
amd64 fs net netncp powerpc
arm geom net80211 netsmb rpc
boot gnu netatalk nfs security
cam i386 netatm nfs4client sparc64
coda i4b netgraph nfsclient sys
compat ia64 netinet nfsserver tools
conf isa netinet6 opencrypto ufs
contrib isofs netipsec pc98 vm
crypto kern netipx pccard
源代码安装好后，我们就可以开始我们的配置内核只旅啦。
二、配置内核文件
其实我们编译内核的关键就是配置内核文件，内核配置文件的目录是在 /usr/src/sys/i386/conf 下，我们进入改目录：
# cd /usr/src/sys/i386/conf
# ls
GENERIC GENERIC.hints NOTES PAE
Makefile OLDCARD gethints.awk
内核配置文件就是GENERIC,我们要配置的就是它,配置包括要开启那些硬件支持，IPv6支持，网卡等等。为了防止我们配置文件配置后有问题，
我们先把它备份一下：
# cp GENERIC GENERIC.bak
然后我们就用任一编辑器打开该文件，可以使用vi，或者ee打开，我为了方便，就使用ee来打开：
# ee GENERIC
打开后就有类似:
# GENERIC -- Generic kernel configuration file for FreeBSD/i386
#
# For more information on this file, please read the handbook section on
# Kernel Configuration Files:
的内容，前面有"#"代表该内容或者选项是注释，那么假如我们要去掉什么选项，就可以在前面加上"#",当然你不需要的选项你也可删除，但是
我推荐不需要的选项在前面加"#"。
关于配置文件详细的修改建议参考FreeBSD中文手册关于内核配置选项的详细说明：
http://www.freebsd.org.cn/snap/doc/zh_CN.GB2312/books/handbook/kernelconfig-config.html
我的配置文件是这么修改的，详细我就不说明了,请查看上面的配置文件参考，应该适合大部分没有ISA,SCSI,RAID等设备的机器。
GENERIC内核配置文件配置：
#########################################################################################
#
# GENERIC -- Generic kernel configuration file for FreeBSD/i386
#
# For more information on this file, please read the handbook section on
# Kernel Configuration Files:
#
# http://www.FreeBSD.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig-config.html
#
# The handbook is also available locally in /usr/share/doc/handbook
# if you've installed the doc distribution, otherwise always see the
# FreeBSD World Wide Web server (http://www.FreeBSD.org/) for the
# latest information.
#
# An exhaustive list of options and more detailed explanations of the
# device lines is also present in the ../../conf/NOTES and NOTES files.
# If you are in doubt as to the purpose or necessity of a line, check first
# in NOTES.
#
# $FreeBSD: src/sys/i386/conf/GENERIC,v 1.394.2.3 2004/01/26 19:42:11 nectar Exp $
machine i386
#cpu I486_CPU
#cpu I586_CPU
cpu I686_CPU
ident GENERIC
#To statically compile in device wiring instead of /boot/device.hints
#hints "GENERIC.hints" #Default places to look for devices.
#makeoptions DEBUG=-g #Build kernel with gdb(1) debug symbols
options SCHED_4BSD #4BSD scheduler
options INET #InterNETworking
#options INET6 #IPv6 communications protocols
options FFS #Berkeley Fast Filesystem
options SOFTUPDATES #Enable FFS soft updates support
options UFS_ACL #Support for access control lists
options UFS_DIRHASH #Improve performance on big directories
options MD_ROOT #MD is a potential root device
#options NFSCLIENT #Network Filesystem Client
#options NFSSERVER #Network Filesystem Server
#options NFS_ROOT #NFS usable as /, requires NFSCLIENT
#options MSDOSFS #MSDOS Filesystem
options CD9660 #ISO 9660 Filesystem
options PROCFS #Process filesystem (requires PSEUDOFS)
options PSEUDOFS #Pseudo-filesystem framework
options COMPAT_43 #Compatible with BSD 4.3 [KEEP THIS!]
options COMPAT_FREEBSD4 #Compatible with FreeBSD4
#options SCSI_DELAY=15000 #Delay (in ms) before probing SCSI
options KTRACE #ktrace(1) support
#options SYSVSHM #SYSV-style shared memory
options SYSVMSG #SYSV-style message queues
options SYSVSEM #SYSV-style semaphores
options _KPOSIX_PRIOR99vY_SCHEDULING #Posix P1003_1B real-time extensions
options KBD_INSTALL_CDEV # install a CDEV entry in /dev
options AHC_REG_PRETTY_PRINT # Print register bitfields in debug
# output. Adds ~128k to driver.
options AHD_REG_PRETTY_PRINT # Print register bitfields in debug
# output. Adds ~215k to driver.
options PFIL_HOOKS # pfil(9) framework
# Debugging for use in -current
#options DDB #Enable the kernel debugger
#options INVARIANTS #Enable calls of extra sanity checking
#options INVARIANT_SUPPORT #Extra sanity checks of internal structures, required by INVARIANTS
#options W99vNESS #Enable checks to detect deadlocks and cycles
#options W99vNESS_SKIPSPIN #Don't run witness on spinlocks for speed
# To make an SMP kernel, the next two are needed
#options SMP # Symmetric MultiProcessor Kernel
#device apic # I/O APIC
device isa
device eisa
device pci
# Floppy drives
device fdc
# ATA and ATAPI devices
device ata
device atadisk # ATA disk drives
#device ataraid # ATA RAID drives
device atapicd # ATAPI CDROM drives
device atapifd # ATAPI floppy drives
#device atapist # ATAPI tape drives
options ATA_STATIC_ID #Static device numbering
# SCSI Controllers
#device ahb # EISA AHA1742 family
#device ahc # AHA2940 and onboard AIC7xxx devices
#device ahd # AHA39320/29320 and onboard AIC79xx devices
#device amd # AMD 53C974 (Tekram DC-390(T))
#device isp # Qlogic family
#device mpt # LSI-Logic MPT-Fusion
#device ncr # NCR/Symbios Logic
#device sym # NCR/Symbios Logic (newer chipsets + those of `ncr')
#device trm # Tekram DC395U/UW/F DC315U adapters
#device adv # Advansys SCSI adapters
#device adw # Advansys wide SCSI adapters
#device aha # Adaptec 154x SCSI adapters
#device aic # Adaptec 15[012]x SCSI adapters, AIC-6[23]60.
#device bt # Buslogic/Mylex MultiMaster SCSI adapters
#device ncv # NCR 53C500
#device nsp # Workbit Ninja SCSI-3
#device stg # TMC 1