8.5. kernel 設定檔解說

Updated for FreeBSD 6.X by Joel Dahl.

kernel 設定檔的內容格式相當簡單。 每一行都包括一個關鍵字,以及一個或多個參數。事實上, 很多行大多只有一個參數。任何以 # 開頭的敘述都將被視為註解而被忽略。 接下來將以在 GENERIC 所出現的順序一一介紹之。 若要看與該平台架構有關的各選項、設備列表, 請參閱與 GENERIC 檔同目錄的 NOTES 檔。 而與平台架構差異較無關的通用部份,則可參閱 /usr/src/sys/conf/NOTES


若為了測試,而需要一份含有所有可用設定的設定檔,那麼請以 root 身份下:

# cd /usr/src/sys/i386/conf && make LINT

下面為 GENERIC 設定檔的範例, 其中包括說明用的註釋。 這例子應該與您機器上的 /usr/src/sys/i386/conf/GENERIC 相當接近。

machine		i386

此處是指機器架構,必須為 alphaamd64i386ia64pc98powerpcsparc64 其中之一。

cpu          I486_CPU
cpu          I586_CPU
cpu          I686_CPU

上面設定是指定要用哪一種 CPU 型號。 也可以同時加上多組 CPU 型號 (比如說萬一不確定是否要用 I586_CPUI686_CPU)。 然而自訂 kernel 的話,建議先確認自己的 CPU 型號,然後只用最適合的那組就好了。 若不確定 CPU 到底是用哪一種, 可以查閱 /var/run/dmesg.boot 的開機訊息以確定。

ident          GENERIC

這是設定該 kernel 名稱為何,可以隨意命名之,像是取名為 MYKERNEL,若是有照先前說明來作大概會取這樣名字。 ident 後面的字串會在開機時顯示,因此若要辨認新 kernel 與常用 kernel 的話,就設定不同組名稱即可(比如在自訂實驗用的 kernel)。

#To statically compile in device wiring instead of /boot/device.hints
#hints          "GENERIC.hints"         # Default places to look for devices.

device.hints(5) 可用來設定各項驅動程式的選項。 開機時 loader(8) 會檢查預設的 /boot/device.hints 設定檔。 使用 hints 選項,就可以把這些 hints 靜態編入 kernel 內。 如此一來就不必在 /boot 內建立 device.hints 檔。

makeoptions     DEBUG=-g          # Build kernel with gdb(1) debug symbols

加上 -g 選項的話,FreeBSD 會在編譯過程加上 debug 用的資訊,透過這選項會讓 gcc(1) 啟用 debug 所會用到的相關資訊。

options          SCHED_4BSD         # 4BSD scheduler

FreeBSD. 傳統所用(並且是預設)的系統 CPU scheduler。 若您不清楚要如何設定 ,請保留這設定。

options          PREEMPTION         # Enable kernel thread preemption

Allows threads that are in the kernel to be preempted by higher priority threads. It helps with interactivity and allows interrupt threads to run sooner rather than waiting.

options          INET              # InterNETworking

Networking support. Leave this in, even if you do not plan to be connected to a network. Most programs require at least loopback networking (i.e., making network connections within your PC), so this is essentially mandatory.

options          INET6             # IPv6 communications protocols

This enables the IPv6 communication protocols.

options          FFS               # Berkeley Fast Filesystem

This is the basic hard drive file system. Leave it in if you boot from the hard disk.

options          SOFTUPDATES       # Enable FFS Soft Updates support

This option enables Soft Updates in the kernel, this will help speed up write access on the disks. Even when this functionality is provided by the kernel, it must be turned on for specific disks. Review the output from mount(8) to see if Soft Updates is enabled for your system disks. If you do not see the soft-updates option then you will need to activate it using the tunefs(8) (for existing file systems) or newfs(8) (for new file systems) commands.

options          UFS_ACL           # Support for access control lists

This option enables kernel support for access control lists. This relies on the use of extended attributes and UFS2, and the feature is described in detail in 節 13.9, “Access Control Lists”. ACLs are enabled by default and should not be disabled in the kernel if they have been used previously on a file system, as this will remove the access control lists, changing the way files are protected in unpredictable ways.

options          UFS_DIRHASH       # Improve performance on big directories

This option includes functionality to speed up disk operations on large directories, at the expense of using additional memory. You would normally keep this for a large server, or interactive workstation, and remove it if you are using FreeBSD on a smaller system where memory is at a premium and disk access speed is less important, such as a firewall.

options          MD_ROOT           # MD is a potential root device

This option enables support for a memory backed virtual disk used as a root device.

options          NFSCLIENT         # Network Filesystem Client
options          NFSSERVER         # Network Filesystem Server
options          NFS_ROOT          # NFS usable as /, requires NFSCLIENT

The network file system. Unless you plan to mount partitions from a UNIX® file server over TCP/IP, you can comment these out.

options          MSDOSFS           # MSDOS Filesystem

The MS-DOS® file system. Unless you plan to mount a DOS formatted hard drive partition at boot time, you can safely comment this out. It will be automatically loaded the first time you mount a DOS partition, as described above. Also, the excellent emulators/mtools software allows you to access DOS floppies without having to mount and unmount them (and does not require MSDOSFS at all).

options          CD9660            # ISO 9660 Filesystem

The ISO 9660 file system for CDROMs. Comment it out if you do not have a CDROM drive or only mount data CDs occasionally (since it will be dynamically loaded the first time you mount a data CD). Audio CDs do not need this file system.

options          PROCFS            # Process filesystem(requires PSEUDOFS)

The process file system. This is a pretend file system mounted on /proc which allows programs like ps(1) to give you more information on what processes are running. Use of PROCFS is not required under most circumstances, as most debugging and monitoring tools have been adapted to run without PROCFS: installs will not mount this file system by default.

options          PSEUDOFS          # Pseudo-filesystem framework

6.X kernels making use of PROCFS must also include support for PSEUDOFS.

options          GEOM_GPT          # GUID Partition Tables.

This option brings the ability to have a large number of partitions on a single disk.

options          COMPAT_43         # Compatible with BSD 4.3 [KEEP THIS!]

Compatibility with 4.3BSD. Leave this in; some programs will act strangely if you comment this out.

options          COMPAT_FREEBSD4   # Compatible with FreeBSD4

This option is required on FreeBSD 5.X i386™ and Alpha systems to support applications compiled on older versions of FreeBSD that use older system call interfaces. It is recommended that this option be used on all i386™ and Alpha systems that may run older applications; platforms that gained support only in 5.X, such as ia64 and Sparc64®, do not require this option.

options          COMPAT_FREEBSD5   # 與 FreeBSD5 相容

此行是 FreeBSD 6.X 及更新的版本若需支援 FreeBSD 5.X 系統呼叫才需要設定。

options          SCSI_DELAY=5000  # Delay (in ms) before probing SCSI

This causes the kernel to pause for 5 seconds before probing each SCSI device in your system. If you only have IDE hard drives, you can ignore this, otherwise you can try to lower this number, to speed up booting. Of course, if you do this and FreeBSD has trouble recognizing your SCSI devices, you will have to raise it again.

options          KTRACE            # ktrace(1) support

This enables kernel process tracing, which is useful in debugging.

options          SYSVSHM           # SYSV-style shared memory

This option provides for System V shared memory. The most common use of this is the XSHM extension in X, which many graphics-intensive programs will automatically take advantage of for extra speed. If you use X, you will definitely want to include this.

options          SYSVMSG           # SYSV-style message queues

Support for System V messages. This option only adds a few hundred bytes to the kernel.

options          SYSVSEM           # SYSV-style semaphores

Support for System V semaphores. Less commonly used but only adds a few hundred bytes to the kernel.


The -p option of the ipcs(1) command will list any processes using each of these System V facilities.

options 	     _KPOSIX_PRIORITY_SCHEDULING # POSIX P1003_1B real-time extensions

Real-time extensions added in the 1993 POSIX®. Certain applications in the Ports Collection use these (such as StarOffice).

options          KBD_INSTALL_CDEV  # install a CDEV entry in /dev

This option is required to allow the creation of keyboard device nodes in /dev.

options          ADAPTIVE_GIANT    # Giant mutex is adaptive.

Giant is the name of a mutual exclusion mechanism (a sleep mutex) that protects a large set of kernel resources. Today, this is an unacceptable performance bottleneck which is actively being replaced with locks that protect individual resources. The ADAPTIVE_GIANT option causes Giant to be included in the set of mutexes adaptively spun on. That is, when a thread wants to lock the Giant mutex, but it is already locked by a thread on another CPU, the first thread will keep running and wait for the lock to be released. Normally, the thread would instead go back to sleep and wait for its next chance to run. If you are not sure, leave this in.


Note that on FreeBSD 8.0-CURRENT and later versions, all mutexes are adaptive by default, unless explicitly set to non-adaptive by compiling with the NO_ADAPTIVE_MUTEXES option. As a result, Giant is adaptive by default now, and the ADAPTIVE_GIANT option has been removed from the kernel configuration.

device          apic               # I/O APIC

The apic device enables the use of the I/O APIC for interrupt delivery. The apic device can be used in both UP and SMP kernels, but is required for SMP kernels. Add options SMP to include support for multiple processors.


apic 只限 i386 架構才有,其他架構則不必加上這行。

device          eisa

Include this if you have an EISA motherboard. This enables auto-detection and configuration support for all devices on the EISA bus.

device          pci

Include this if you have a PCI motherboard. This enables auto-detection of PCI cards and gatewaying from the PCI to ISA bus.

# Floppy drives
device          fdc

This is the floppy drive controller.

# ATA and ATAPI devices
device          ata

This driver supports all ATA and ATAPI devices. You only need one device ata line for the kernel to detect all PCI ATA/ATAPI devices on modern machines.

device          atadisk                 # ATA disk drives

This is needed along with device ata for ATA disk drives.

device          ataraid                 # ATA RAID drives

This is needed along with device ata for ATA RAID drives.

device          atapicd                 # ATAPI CDROM drives

This is needed along with device ata for ATAPI CDROM drives.

device          atapifd                 # ATAPI floppy drives

This is needed along with device ata for ATAPI floppy drives.

device          atapist                 # ATAPI tape drives

This is needed along with device ata for ATAPI tape drives.

options         ATA_STATIC_ID           # Static device numbering

This makes the controller number static; without this, the device numbers are dynamically allocated.

# SCSI Controllers
device          ahb        # EISA AHA1742 family
device          ahc        # AHA2940 and onboard AIC7xxx devices
options         AHC_REG_PRETTY_PRINT    # Print register bitfields in debug
                                        # output.  Adds ~128k to driver.
device          ahd        # AHA39320/29320 and onboard AIC79xx devices
options         AHD_REG_PRETTY_PRINT    # Print register bitfields in debug
				        # output.  Adds ~215k to driver.
device          amd        # AMD 53C974 (Teckram DC-390(T))
device          isp        # Qlogic family
device          ispfw      # Firmware for QLogic HBAs- normally a module
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 controllers. Comment out any you do not have in your system. If you have an IDE only system, you can remove these altogether. The *_REG_PRETTY_PRINT lines are debugging options for their respective drivers.

# 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)

SCSI peripherals. Again, comment out any you do not have, or if you have only IDE hardware, you can remove them completely.


The USB umass(4) driver and a few other drivers use the SCSI subsystem even though they are not real SCSI devices. Therefore make sure not to remove SCSI support, if any such drivers are included in the kernel configuration.

# RAID controllers interfaced to the SCSI subsystem
device          amr        # AMI MegaRAID
device          arcmsr     # Areca SATA II RAID
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          hptmv      # Highpoint RocketRAID 182x
device          rr232x     # Highpoint RocketRAID 232x
device          iir        # Intel Integrated RAID
device          ips        # IBM (Adaptec) ServeRAID
device          mly        # Mylex AcceleRAID/eXtremeRAID
device          twa        # 3ware 9000 series PATA/SATA RAID

# RAID controllers
device          aac        # Adaptec FSA RAID
device          aacp       # SCSI passthrough for aac (requires CAM)
device          ida        # Compaq Smart RAID
device          mfi        # LSI MegaRAID SAS
device          mlx        # Mylex DAC960 family
device          pst        # Promise Supertrak SX6000
device          twe        # 3ware ATA RAID

Supported RAID controllers. If you do not have any of these, you can comment them out or remove them.

# atkbdc0 controls both the keyboard and the PS/2 mouse
device          atkbdc     # AT keyboard controller

The keyboard controller (atkbdc) provides I/O services for the AT keyboard and PS/2 style pointing devices. This controller is required by the keyboard driver (atkbd) and the PS/2 pointing device driver (psm).

device          atkbd      # AT keyboard

The atkbd driver, together with atkbdc controller, provides access to the AT 84 keyboard or the AT enhanced keyboard which is connected to the AT keyboard controller.

device          psm        # PS/2 mouse

Use this device if your mouse plugs into the PS/2 mouse port.

device          kbdmux        # keyboard multiplexer

多重鍵盤的支援。 若不打算同時接多組鍵盤的話, 那麼若要移除該行也沒關係。

device          vga        # VGA video card driver

The video card driver.

device          splash     # Splash screen and screen saver support

Splash screen at start up! Screen savers require this too.

# syscons is the default console driver, resembling an SCO console
device          sc

sc is the default console driver and resembles a SCO console. Since most full-screen programs access the console through a terminal database library like termcap, it should not matter whether you use this or vt, the VT220 compatible console driver. When you log in, set your TERM variable to scoansi if full-screen programs have trouble running under this console.

# 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

This is a VT220-compatible console driver, backward compatible to VT100/102. It works well on some laptops which have hardware incompatibilities with sc. Also set your TERM variable to vt100 or vt220 when you log in. This driver might also prove useful when connecting to a large number of different machines over the network, where termcap or terminfo entries for the sc device are often not available —— vt100 should be available on virtually any platform.

device          agp

Include this if you have an AGP card in the system. This will enable support for AGP, and AGP GART for boards which have these features.

# Power management support (see NOTES for more options)
#device          apm

Advanced Power Management support. Useful for laptops, although in FreeBSD 5.X and above this is disabled in GENERIC by default.

# Add suspend/resume support for the i8254.
device           pmtimer

Timer device driver for power management events, such as APM and ACPI.

# PCCARD (PCMCIA) support
# PCMCIA and cardbus bridge support
device          cbb               # cardbus (yenta) bridge
device          pccard            # PC Card (16-bit) bus
device          cardbus           # CardBus (32-bit) bus

PCMCIA support. You want this if you are using a laptop.

# Serial (COM) ports
device          sio               # 8250, 16[45]50 based serial ports

These are the serial ports referred to as COM ports in the MS-DOS®/Windows® world.


If you have an internal modem on COM4 and a serial port at COM2, you will have to change the IRQ of the modem to 2 (for obscure technical reasons, IRQ2 = IRQ 9) in order to access it from FreeBSD. If you have a multiport serial card, check the manual page for sio(4) for more information on the proper values to add to your /boot/device.hints. Some video cards (notably those based on S3 chips) use IO addresses in the form of 0x*2e8, and since many cheap serial cards do not fully decode the 16-bit IO address space, they clash with these cards making the COM4 port practically unavailable.

Each serial port is required to have a unique IRQ (unless you are using one of the multiport cards where shared interrupts are supported), so the default IRQs for COM3 and COM4 cannot be used.

# Parallel port
device          ppc

This is the ISA-bus parallel port interface.

device          ppbus      # Parallel port bus (required)

Provides support for the parallel port bus.

device          lpt        # Printer

Support for parallel port printers.


All three of the above are required to enable parallel printer support.

device          plip       # TCP/IP over parallel

This is the driver for the parallel network interface.

device          ppi        # Parallel port interface device

The general-purpose I/O (geek port) + IEEE1284 I/O.

#device         vpo        # Requires scbus and da

This is for an Iomega Zip drive. It requires scbus and da support. Best performance is achieved with ports in EPP 1.9 mode.

#device         puc

Uncomment this device if you have a dumb serial or parallel PCI card that is supported by the puc(4) glue driver.

# PCI Ethernet NICs.
device          de         # DEC/Intel DC21x4x (Tulip)
device          em         # Intel PRO/1000 adapter Gigabit Ethernet Card
device          ixgb       # Intel PRO/10GbE Ethernet Card
device          txp        # 3Com 3cR990 (Typhoon)
device          vx         # 3Com 3c590, 3c595 (Vortex)

Various PCI network card drivers. Comment out or remove any of these not present in your system.

# 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

MII bus support is required for some PCI 10/100 Ethernet NICs, namely those which use MII-compliant transceivers or implement transceiver control interfaces that operate like an MII. Adding device miibus to the kernel config pulls in support for the generic miibus API and all of the PHY drivers, including a generic one for PHYs that are not specifically handled by an individual driver.

device          bce        # Broadcom BCM5706/BCM5708 Gigabit Ethernet
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          lge        # Level 1 LXT1001 gigabit ethernet
device          msk        # Marvell/SysKonnect Yukon II Gigabit Ethernet
device          nge        # NatSemi DP83820 gigabit ethernet
device          nve        # nVidia nForce MCP on-board Ethernet Networking
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 & SK-982x gigabit Ethernet
device          ste        # Sundance ST201 (D-Link DFE-550TX)
device          stge       # Sundance/Tamarack TC9021 gigabit Ethernet
device          ti         # Alteon Networks Tigon I/II gigabit Ethernet
device          tl         # Texas Instruments ThunderLAN
device          tx         # SMC EtherPower II (83c170 EPIC)
device          vge        # VIA VT612x gigabit ethernet
device          vr         # VIA Rhine, Rhine II
device          wb         # Winbond W89C840F
device          xl         # 3Com 3c90x (Boomerang, Cyclone)

Drivers that use the MII bus controller code.

# 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

ISA Ethernet drivers. See /usr/src/sys/i386/conf/NOTES for details of which cards are supported by which driver.

# Wireless NIC cards
device          wlan       # 802.11 support

對 802.11 標準的支援。 若要無線上網,則需加上這行。

device          wlan_wep        # 802.11 WEP support
device          wlan_ccmp       # 802.11 CCMP support
device          wlan_tkip       # 802.11 TKIP support

對 802.11 加密設備的支援。 若要安全加密以及 802.11i 安全協定, 則需加上這行。

device          an         # Aironet 4500/4800 802.11 wireless NICs.
device          ath             # Atheros pci/cardbus NIC's
device          ath_hal         # Atheros HAL (Hardware Access Layer)
device          ath_rate_sample # SampleRate tx rate control for ath
device          an         # Aironet 4500/4800 802.11 wireless NICs.
device          awi        # BayStack 660 and others
device          ral        # Ralink Technology RT2500 wireless NICs.
device          wi         # WaveLAN/Intersil/Symbol 802.11 wireless NICs.
#device         wl         # Older non 802.11 Wavelan wireless NIC.

Support for various wireless cards.

# Pseudo devices
device   loop          # Network loopback

This is the generic loopback device for TCP/IP. If you telnet or FTP to localhost (a.k.a. it will come back at you through this device. This is mandatory.

device   random        # Entropy device

Cryptographically secure random number generator.

device   ether         # Ethernet support

ether is only needed if you have an Ethernet card. It includes generic Ethernet protocol code.

device   sl            # Kernel SLIP

sl is for SLIP support. This has been almost entirely supplanted by PPP, which is easier to set up, better suited for modem-to-modem connection, and more powerful.

device   ppp           # Kernel PPP

This is for kernel PPP support for dial-up connections. There is also a version of PPP implemented as a userland application that uses tun and offers more flexibility and features such as demand dialing.

device   tun           # Packet tunnel.

This is used by the userland PPP software. See the PPP section of this book for more information.

device   pty           # Pseudo-ttys (telnet etc)

This is a pseudo-terminal or simulated login port. It is used by incoming telnet and rlogin sessions, xterm, and some other applications such as Emacs.

device   md            # Memory disks

Memory disk pseudo-devices.

device   gif           # IPv6 and IPv4 tunneling

This implements IPv6 over IPv4 tunneling, IPv4 over IPv6 tunneling, IPv4 over IPv4 tunneling, and IPv6 over IPv6 tunneling. The gif device is auto-cloning, and will create device nodes as needed.

device   faith         # IPv6-to-IPv4 relaying (translation)

This pseudo-device captures packets that are sent to it and diverts them to the IPv4/IPv6 translation daemon.

# 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

This is the Berkeley Packet Filter. This pseudo-device allows network interfaces to be placed in promiscuous mode, capturing every packet on a broadcast network (e.g., an Ethernet). These packets can be captured to disk and or examined with the tcpdump(1) program.


The bpf(4) device is also used by dhclient(8) to obtain the IP address of the default router (gateway) and so on. If you use DHCP, leave this uncommented.

# 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          ulpt          # Printer
device          umass         # Disks/Mass storage - Requires scbus and da
device          ums           # Mouse
device          ural          # Ralink Technology RT2500USB wireless NICs
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          cdce          # Generic USB over Ethernet
device          cue           # CATC USB Ethernet
device          kue           # Kawasaki LSI USB Ethernet
device          rue           # RealTek RTL8150 USB Ethernet

Support for various USB devices.

# FireWire support
device          firewire      # FireWire bus code
device          sbp           # SCSI over FireWire (Requires scbus and da)
device          fwe           # Ethernet over FireWire (non-standard!)

Support for various Firewire devices.

For more information and additional devices supported by FreeBSD, see /usr/src/sys/i386/conf/NOTES.

8.5.1. Large Memory Configurations (PAE)

Large memory configuration machines require access to more than the 4 gigabyte limit on User+Kernel Virtual Address (KVA) space. Due to this limitation, Intel added support for 36-bit physical address space access in the Pentium® Pro and later line of CPUs.

The Physical Address Extension (PAE) capability of the Intel® Pentium® Pro and later CPUs allows memory configurations of up to 64 gigabytes. FreeBSD provides support for this capability via the PAE kernel configuration option, available in all current release versions of FreeBSD. Due to the limitations of the Intel memory architecture, no distinction is made for memory above or below 4 gigabytes. Memory allocated above 4 gigabytes is simply added to the pool of available memory.

To enable PAE support in the kernel, simply add the following line to your kernel configuration file:

options		    PAE


The PAE support in FreeBSD is only available for Intel® IA-32 processors. It should also be noted, that the PAE support in FreeBSD has not received wide testing, and should be considered beta quality compared to other stable features of FreeBSD.

PAE support in FreeBSD has a few limitations:

  • A process is not able to access more than 4 gigabytes of VM space.

  • KLD modules cannot be loaded into a PAE enabled kernel, due to the differences in the build framework of a module and the kernel.

  • Device drivers that do not use the bus_dma(9) interface will cause data corruption in a PAE enabled kernel and are not recommended for use. For this reason, a PAE kernel configuration file is provided in FreeBSD which excludes all drivers not known to work in a PAE enabled kernel.

  • Some system tunables determine memory resource usage by the amount of available physical memory. Such tunables can unnecessarily over-allocate due to the large memory nature of a PAE system. One such example is the kern.maxvnodes sysctl, which controls the maximum number of vnodes allowed in the kernel. It is advised to adjust this and other such tunables to a reasonable value.

  • It might be necessary to increase the kernel virtual address (KVA) space or to reduce the amount of specific kernel resource that is heavily used (see above) in order to avoid KVA exhaustion. The KVA_PAGES kernel option can be used for increasing the KVA space.

For performance and stability concerns, it is advised to consult the tuning(7) manual page. The pae(4) manual page contains up-to-date information on FreeBSD's PAE support.

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