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_PRIORITY_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 WITNESS #Enable checks to detect deadlocks and cycles
#options WITNESS_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 18C30/18C50
# SCSI peripherals
device scbus # SCSI bus (required for SCSI)
#device ch # SCSI media changers
#device da # Direct Access (disks)
#device sa # Sequential Access (tape etc)
#device cd # CD
#device pass # Passthrough device (direct SCSI access)
#device ses # SCSI Environmental Services (and SAF-TE)
# RAID controllers interfaced to the SCSI subsystem
#device amr # AMI MegaRAID
#device asr # DPT SmartRAID V, VI and Adaptec SCSI RAID
#device ciss # Compaq Smart RAID 5*
#device dpt # DPT Smartcache III, IV - See NOTES for options
#device iir # Intel Integrated RAID
#device ips # IBM (Adaptec) ServeRAID
#device mly # Mylex AcceleRAID/eXtremeRAID
# RAID controllers
#device aac # Adaptec FSA RAID
#device aacp # SCSI passthrough for aac (requires CAM)
#device ida # Compaq Smart RAID
#device mlx # Mylex DAC960 family
#device pst # Promise Supertrak SX6000
#device twe # 3ware ATA RAID
# atkbdc0 controls both the keyboard and the PS/2 mouse
device atkbdc # AT keyboard controller
device atkbd # AT keyboard
device psm # PS/2 mouse
device vga # VGA video card driver
device splash # Splash screen and screen saver support
# syscons is the default console driver, resembling an SCO console
device sc
# Enable this for the pcvt (VT220 compatible) console driver
#device vt
#options XSERVER # support for X server on a vt console
#options FAT_CURSOR # start with block cursor
device agp # support several AGP chipsets
# Floating point support - do not disable.
device npx
# Power management support (see NOTES for more options)
#device apm
# Add suspend/resume support for the i8254.
#device pmtimer
# PCCARD (PCMCIA) support
# Pcmcia and cardbus bridge support
#device cbb # cardbus (yenta) bridge
#device pcic # ExCA ISA and PCI bridges
#device pccard # PC Card (16-bit) bus
#device cardbus # CardBus (32-bit) bus
# Serial (COM) ports
#device sio # 8250, 16[45]50 based serial ports
# Parallel port
device ppc
device ppbus # Parallel port bus (required)
#device lpt # Printer
device plip # TCP/IP over parallel
device ppi # Parallel port interface device
#device vpo # Requires scbus and da
# If you've got a
"dumb" serial or parallel PCI card that is# supported by the puc(4) glue driver, uncomment the following
# line to enable it (connects to the sio and/or ppc drivers):
#device puc
# PCI Ethernet NICs.
#device de # DEC/Intel DC21x4x (``Tulip'')
device em # Intel PRO/1000 adapter Gigabit Ethernet Card
#device txp # 3Com 3cR990 (``Typhoon'')
#device vx # 3Com 3c590, 3c595 (``Vortex'')
# PCI Ethernet NICs that use the common MII bus controller code.
# NOTE: Be sure to keep the 'device miibus' line in order to use these NICs!
device miibus # MII bus support
#device bfe # Broadcom BCM440x 10/100 ethernet
#device bge # Broadcom BCM570xx Gigabit Ethernet
#device dc # DEC/Intel 21143 and various workalikes
device fxp # Intel EtherExpress PRO/100B (82557, 82558)
#device pcn # AMD Am79C97x PCI 10/100 (precedence over 'lnc')
#device re # RealTek 8139C+/8169/8169S/8110S
#device rl # RealTek 8129/8139
#device sf # Adaptec AIC-6915 (``Starfire'')
#device sis # Silicon Integrated Systems SiS 900/SiS 7016
#device sk # SysKonnect SK-984x and SK-982x gigabit ethernet
#device ste # Sundance ST201 (D-Link DFE-550TX)
#device ti # Alteon Networks Tigon I/II gigabit ethernet
#device tl # Texas Instruments ThunderLAN
#device tx # SMC EtherPower II (83c170 ``EPIC'')
#device vr # VIA Rhine, Rhine II
#device wb # Winbond W89C840F
#device xl # 3Com 3c90x (``Boomerang'', ``Cyclone'')
# ISA Ethernet NICs. pccard nics included.
#device cs # Crystal Semiconductor CS89x0 NIC
# 'device ed' requires 'device miibus'
#device ed # NE[12]000, SMC Ultra, 3c503, DS8390 cards
#device ex # Intel EtherExpress Pro/10 and Pro/10+
#device ep # Etherlink III based cards
#device fe # Fujitsu MB8696x based cards
#device ie # EtherExpress 8/16, 3C507, StarLAN 10 etc.
#device lnc # NE2100, NE32-VL Lance Ethernet cards
#device sn # SMC's 9000 series of ethernet chips
#device xe # Xircom pccard ethernet
# ISA devices that use the old ISA shims
#device le
# Wireless NIC cards
#device wlan # 802.11 support
#device an # Aironet 4500/4800 802.11 wireless NICs.
#device awi # BayStack 660 and others
#device wi # WaveLAN/Intersil/Symbol 802.11 wireless NICs.
#device wl # Older non 802.11 Wavelan wireless NIC.
# Pseudo devices - the number indicates how many units to allocate.
device random # Entropy device
device loop # Network loopback
device ether # Ethernet support
device sl # Kernel SLIP
device ppp # Kernel PPP
device tun # Packet tunnel.
device pty # Pseudo-ttys (telnet etc)
#device md # Memory "disks"
#device gif # IPv6 and IPv4 tunneling
#device faith # IPv6-to-IPv4 relaying (translation)
# The `bpf' device enables the Berkeley Packet Filter.
# Be aware of the administrative consequences of enabling this!
device bpf # Berkeley packet filter
# USB support
device uhci # UHCI PCI->USB interface
device ohci # OHCI PCI->USB interface
device usb # USB Bus (required)
#device udbp # USB Double Bulk Pipe devices
device ugen # Generic
device uhid # "Human Interface Devices"
device ukbd # Keyboard
#device ulpt # Printer
device umass # Disks/Mass storage - Requires scbus and da
device ums # Mouse
device urio # Diamond Rio 500 MP3 player
#device uscanner # Scanners
# USB Ethernet, requires mii
device aue # ADMtek USB ethernet
device axe # ASIX Electronics USB ethernet
device cue # CATC USB ethernet
device kue # Kawasaki LSI USB ethernet
# FireWire support
#device firewire # FireWire bus code
#device sbp # SCSI over FireWire (Requires scbus and da)
#device fwe # Ethernet over FireWire (non-standard!)
#
##############################################################################################
配置完成后，按ESC键，然后提示按"a"后再按"a"就能够把配置文件保存。
三、编译内核：
内核文件配置好以后就能够进行内核的编译了，一般有两种编译的方式，我们采取传统的编译方式。
我们进入 /usr/src/sys/i386/conf 目录：
# cd /usr/src/sys/i386/conf
运行config命令来产生内核源代码。
# /usr/sbin/config GENERIC
切换到建立内核的目录。
# cd ../compile/GENERIC
编译内核
# make depend
# make
安装新内核。
# make install
OK,到这里，如果中间没有出现任何错误的话，那么我们的内核心就已经安装成功了，reboot后就能够使用我们的新编译的内核了。如果出现了
错误，那么请检查你的步骤是否跟上面的步骤一致，同时要检查你的内核配置文件配置的是否正确，一般编译不成功都是内核文件配置的问题
，请仔细检查。
四、新内核无法启动：
如果你编译的内核无法启动，那么可以使用以前的内核来启动。一般编译一次内核后，都会把把旧内核备份，一般/boot/kernel.old就是旧内
核的备份，出现问题就能够使用久内核来启动。
具体实现方法是：
你可以在系统启动进行10计数的时候按Enter以外的任何键， 接着键入unload命令，再键入boot kernel.old，或者任何能正常启动的内核名字
，然后就能够正常启动系统，进入系统后再重新进行内核的配置工作。
注意： 你不能仅依靠kernel.old，因为当你安装一个新内核时， kernel.old会被上一次安装的内核所覆盖，所以你要么自己手动备份一个能
正常启动的内核，或者注意你的kernel.old是否能正常启动的内核。
结束
好，到这里，我们的编译内核的工作就完成了，来然我们来看看我们的内核是否已经减小了：
# ls -l /boot/kernel/kernel
-r-xr-xr-x 1 root wheel 3090929 Sep 1 21:30 /boot/kernel/kernel
呵呵，明显把原来的5940286变成了现在的3090929，虽然没有变小一倍，但是也变小好多啦，以后系统又更快啦！
注意： 本文是在FreeBSD5.2.1的环境下进行的，不一定适合其他版本，特别是FreeBSD4.X版本，如果出现问题，请参考手册。
附：
FreeBSD中文手册: http://www.freebsd.org.cn/snap/doc/zh_CN.GB2312/books/handbook/index.html
内核配置参考: http://www.freebsd.org.cn/snap/doc/zh_CN.GB2312/bos/handbook/kernelconfig.html