Compiling FreeBSD Kernel
Since my College days, when I was introduced to Unixes, I love to have my own compiled kernel. It gave me that special feeling of "my machine is better than yours". Besides, a custom kernel serves as an interesting conversation topic with geeks. Just ask them "What is your kernel size ?". This would keep the Geek busy for sometime where he would explain all the great things he did to the kernel to optimize his box. If you happen to be a geek yourself, this is a great question to put forward to Newbies. They’ll never trouble you with stories of their nephew or kid sister after this 
A custom kernel boots up the system faster, it takes up lesser memory space and one can install drivers for special hardware not supported by a default kernel.
Before we get down to compiling our own kernel, it is a wonderful idea to backup the present kernel.
# cd /boot
# cp -pvr kernel kernel.orig
The original generic kernel is now stored as kernel.orig. In case the kernel does not boot (which has happened with me many times) boot up with the generic kernel and try again. Booting any other kernel is simple. At the boot time goto to the boot prompt and simple say "boot kernel.orig" and the default kernel would boot up.
The kernel sources are located in the /usr/src/sys directory. If not installed, the kernel sources can be installed with the following commands. First insert the FreeBSD install CD in your CD drive.
# mount /cdrom
# mkdir -p /usr/src/sys
# ln -s /usr/src/sys /sys
# cat /cdrom/src/ssys.[a-d]* | tar -xzvf -
Then go to the director where the configuration files are located.
#cd /sys/i386/conf
In this directory the GENERIC kernel configuration file is located. Copy this file and it as your custom name.
#cp -pvr GENERIC BABYBSD
# vi BABYBSD
First change the ident value from GENERIC to BABYBSD (or whatever name you prefer). I have given my sample configuration file at the end of this article. The options are quite easy to understand and a good starting point would be the FreeBSD Handbook which has documented most of the kernel options.
You can also go to a shell and run the command #make LINT in this directory which would create a file called LINT which has all the options supported by the kernel listed.
After this stage just run the following commands
#/usr/sbin/config BABYBSD
#cd ../compile/BABYBSD
#make depend
#make
#make install
Please note that you must be root to run the make install. Reboot and the kernel should boot with your shining new kernel. 
# My Sample Kernel Configuration File for FreeBSD/i386
machine       i386
cpu            I586_CPU
cpu            I686_CPU
ident          BABYBSD
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    NTFS
options    CD9660           # ISO 9660 Filesystem
options    PROCFS           # Process filesystem (requires PSEUDOFS)
options    PSEUDOFS       # Pseudo-filesystem framework
options    GEOM_GPT       # GUID Partition Tables.
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
options    AHD_REG_PRETTY_PRINT   # Print register bitfields in debug
options    ADAPTIVE_GIANT       # Giant mutex is adaptive.
device      apic           # I/O APIC
# Bus support. Do not remove isa, even if you have no isa slots
device       isa
device       eisa
device       pci
# Floppy drives
device       fdc
# ATA and ATAPI devices
device       ata
device       atadisk       # ATA disk drives
device       atapicd       # ATAPI CDROM drives
options    ATA_STATIC_ID   # Static device numbering
# SCSI peripherals
device       scbus       # SCSI bus (required for SCSI)
device       ch       # SCSI media changers
device       da       # Direct Access (disks)
device       cd       # CD
device       pass       # Passthrough device (direct SCSI access)
device       ses       # SCSI Environmental Services (and SAF-TE)
# 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
# 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       rl       # RealTek 8129/8139
# Wireless NIC cards
device       wlan       # 802.11 support
# Pseudo devices.
device       loop       # Network loopback
device       mem       # Memory and kernel memory devices
device       io       # I/O device
device       random       # Entropy device
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!
# Note that ‘bpf’ is required for DHCP.
device       bpf       # Berkeley packet filter
# USB support
device       uhci       # UHCI PCI->USB interface
device       ohci       # OHCI PCI->USB interface
device       ehci       # EHCI PCI->USB interface (USB 2.0)
device       usb       # USB Bus (required)
#device       udbp       # USB Double Bulk Pipe devices
device       ugen       # Generic
device       uhid       # "Human Interface Devices"
device       ukbd       # Keyboard
device       umass       # Disks/Mass storage – Requires scbus and da
device       ums       # Mouse
# Specific to my Machine
device        atapicam   # for CD Writer to be detected by K3B and other CD writing software
device         sound         # To Enable Sound
device         "snd_ad1816"
device         "snd_als4000"
device         snd_cmi
device         "snd_cs4281"
device         snd_csa
device         "snd_ds1"
device         "snd_emu10k1"
device         "snd_es137x"
device         snd_ess
device         "snd_fm801"
device         snd_gusc
device         snd_ich
device         snd_maestro
device         "snd_maestro3"
device         snd_mss
device         snd_neomagic
device         "snd_sb16"
device         snd_sbc
device         snd_solo
device         "snd_t4dwave"
device         "snd_via8233"
device         "snd_via82c686"
device         snd_vibes
device         snd_uaudio
device pcii
device         acpi
device         acpi_asus
device         acpi_video
device         cbb            # cardbus (yenta) bridge
device         pccard         # PC Card (16-bit) bus
device         cardbus        # CardBus (32-bit) bus
options    DEVICE_POLLING
options    HZ=1000
options       PERFMON
device         apm_saver      # Requires APM
options        AUTO_EOI_1
options       AUTO_EOI_2
options        VESA
Article by Amarjyoti Krishnan of BobCare.com
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