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          bootparam - introduction to boot time parameters of the
          Linux kernel

          The Linux kernel accepts certain 'command-line options' or
          'boot time parameters' at the moment it is started.  In
          general, this is used to supply the kernel with information
          about hardware parameters that the kernel would not be able
          to determine on its own, or to avoid/override the values
          that the kernel would otherwise detect.

          When the kernel is booted directly by the BIOS, you have no
          opportunity to specify any parameters.  So, in order to take
          advantage of this possibility you have to use a boot loader
          that is able to pass parameters, such as GRUB.

        The argument list
          The kernel command line is parsed into a list of strings
          (boot arguments) separated by spaces.  Most of the boot
          arguments have the form:


          where 'name' is a unique keyword that is used to identify
          what part of the kernel the associated values (if any) are
          to be given to.  Note the limit of 10 is real, as the
          present code handles only 10 comma separated parameters per
          keyword.  (However, you can reuse the same keyword with up
          to an additional 10 parameters in unusually complicated
          situations, assuming the setup function supports it.)

          Most of the sorting is coded in the kernel source file
          init/main.c. First, the kernel checks to see if the argument
          is any of the special arguments 'root=', 'nfsroot=', 'nfsad-
          drs=', 'ro', 'rw', 'debug' or 'init'.  The meaning of these
          special arguments is described below.

          Then it walks a list of setup functions to see if the speci-
          fied argument string (such as 'foo') has been associated
          with a setup function ('foo_setup()') for a particular
          device or part of the kernel.  If you passed the kernel the
          line foo=3,4,5,6 then the kernel would search the bootsetups
          array to see if 'foo' was registered.  If it was, then it
          would call the setup function associated with 'foo'
          (foo_setup()) and hand it the arguments 3, 4, 5, and 6 as
          given on the kernel command line.

          Anything of the form 'foo=bar' that is not accepted as a
          setup function as described above is then interpreted as an

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          environment variable to be set.  A (useless?) example would
          be to use 'TERM=vt100' as a boot argument.

          Any remaining arguments that were not picked up by the ker-
          nel and were not interpreted as environment variables are
          then passed onto PID 1, which is usually the init(1) pro-
          gram.  The most common argument that is passed to the init
          process is the word 'single' which instructs it to boot the
          computer in single user mode, and not launch all the usual
          daemons.  Check the manual page for the version of init(1)
          installed on your system to see what arguments it accepts.

        General non-device-specific boot arguments
               This sets the initial command to be executed by the
               kernel.  If this is not set, or cannot be found, the
               kernel will try /sbin/init, then /etc/init, then
               /bin/init, then /bin/sh and panic if all of this fails.

               This sets the NFS boot address to the given string.
               This boot address is used in case of a net boot.

               This sets the NFS root name to the given string.  If
               this string does not begin with '/' or ',' or a digit,
               then it is prefixed by '/tftpboot/'.  This root name is
               used in case of a net boot.

               This argument tells the kernel what device is to be
               used as the root filesystem while booting.  The default
               of this setting is determined at compile time, and usu-
               ally is the value of the root device of the system that
               the kernel was built on.  To override this value, and
               select the second floppy drive as the root device, one
               would use 'root=/dev/fd1'.

               The root device can be specified symbolically or numer-
               ically.  A symbolic specification has the form
               /dev/XXYN, where XX designates the device type (e.g.,
               'hd' for ST-506 compatible hard disk, with Y in
               'a'en'd'; 'sd' for SCSI compatible disk, with Y in
               'a'en'e'), Y the driver letter or number, and N the num-
               ber (in decimal) of the partition on this device.

               Note that this has nothing to do with the designation
               of these devices on your filesystem.  The '/dev/' part
               is purely conventional.

               The more awkward and less portable numeric specifica-
               tion of the above possible root devices in major/minor

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               format is also accepted.  (For example, /dev/sda3 is
               major 8, minor 3, so you could use 'root=0x803' as an

               This parameter sets the delay (in seconds) to pause
               before attempting to mount the root filesystem.

               This parameter sets the mount option string for the
               root filesystem (see also fstab(5)).

               The 'rootfstype' option tells the kernel to mount the
               root filesystem as if it where of the type specified.
               This can be useful (for example) to mount an ext3
               filesystem as ext2 and then remove the journal in the
               root filesystem, in fact reverting its format from ext3
               to ext2 without the need to boot the box from alternate

          'ro' and 'rw'
               The 'ro' option tells the kernel to mount the root
               filesystem as 'read-only' so that filesystem consis-
               tency check programs (fsck) can do their work on a qui-
               escent filesystem.  No processes can write to files on
               the filesystem in question until it is 'remounted' as
               read/write capable, for example, by 'mount -w -n -o
               remount /'.  (See also mount(8).)

               The 'rw' option tells the kernel to mount the root
               filesystem read/write.  This is the default.

               This tells the kernel the location of the suspend-to-
               disk data that you want the machine to resume from
               after hibernation.  Usually, it is the same as your
               swap partition or file.  Example:


               This is used to protect I/O port regions from probes.
               The form of the command is:


               In some machines it may be necessary to prevent device
               drivers from checking for devices (auto-probing) in a
               specific region.  This may be because of hardware that
               reacts badly to the probing, or hardware that would be
               mistakenly identified, or merely hardware you don't

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               want the kernel to initialize.

               The reserve boot-time argument specifies an I/O port
               region that shouldn't be probed.  A device driver will
               not probe a reserved region, unless another boot argu-
               ment explicitly specifies that it do so.

               For example, the boot line

                   reserve=0x300,32  blah=0x300

               keeps all device drivers except the driver for 'blah'
               from probing 0x300-0x31f.

               By default, the kernel will not reboot after a panic,
               but this option will cause a kernel reboot after N sec-
               onds (if N is greater than zero).  This panic timeout
               can also be set by

                   echo N > /proc/sys/kernel/panic

               Since Linux 2.0.22, a reboot is by default a cold
               reboot.  One asks for the old default with
               'reboot=warm'.  (A cold reboot may be required to reset
               certain hardware, but might destroy not yet written
               data in a disk cache.  A warm reboot may be faster.)
               By default, a reboot is hard, by asking the keyboard
               controller to pulse the reset line low, but there is at
               least one type of motherboard where that doesn't work.
               The option 'reboot=bios' will instead jump through the

          'nosmp' and 'maxcpus=N'
               (Only when __SMP__ is defined.)  A command-line option
               of 'nosmp' or 'maxcpus=0' will disable SMP activation
               entirely; an option 'maxcpus=N' limits the maximum num-
               ber of CPUs activated in SMP mode to N.

        Boot arguments for use by kernel
               Kernel messages are handed off to a daemon (e.g.,
               klogd(8) or similar) so that they may be logged to
               disk.  Messages with a priority above console_loglevel
               are also printed on the console.  (For a discussion of
               log levels, see syslog(2).)  By default,
               console_loglevel is set to log messages at levels
               higher than KERN_DEBUG.  This boot argument will cause
               the kernel to also print messages logged at level
               KERN_DEBUG.  The console loglevel can also be set on a
               booted system via the /proc/sys/kernel/printk file

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               (described in syslog(2)), the syslog(2)
               SYSLOG_ACTION_CONSOLE_LEVEL operation, or dmesg(8).

               It is possible to enable a kernel profiling function,
               if one wishes to find out where the kernel is spending
               its CPU cycles.  Profiling is enabled by setting the
               variable prof_shift to a nonzero value.  This is done
               either by specifying CONFIG_PROFILE at compile time, or
               by giving the 'profile=' option.  Now the value that
               prof_shift gets will be N, when given, or
               CONFIG_PROFILE_SHIFT, when that is given, or 2, the
               default.  The significance of this variable is that it
               gives the granularity of the profiling: each clock
               tick, if the system was executing kernel code, a
               counter is incremented:

                   profile[address >> prof_shift]++;

               The raw profiling information can be read from
               /proc/profile. Probably you'll want to use a tool such
               as readprofile.c to digest it.  Writing to
               /proc/profile will clear the counters.

        Boot arguments for ramdisk use
          (Only if the kernel was compiled with CONFIG_BLK_DEV_RAM.)
          In general it is a bad idea to use a ramdisk under Linux-the
          system will use available memory more efficiently itself.
          But while booting, it is often useful to load the floppy
          contents into a ramdisk.  One might also have a system in
          which first some modules (for filesystem or hardware) must
          be loaded before the main disk can be accessed.

               In Linux 1.3.48, ramdisk handling was changed drasti-
               cally.  Earlier, the memory was allocated statically,
               and there was a 'ramdisk=N' parameter to tell its size.
               (This could also be set in the kernel image at compile
               time.)  These days ram disks use the buffer cache, and
               grow dynamically.  For a lot of information on the cur-
               rent ramdisk setup, see the kernel source file
               (Documentation/ramdisk.txt in older kernels).

               There are four parameters, two boolean and two inte-

               If N=1, do load a ramdisk.  If N=0, do not load a ram-
               disk.  (This is the default.)

               If N=1, do prompt for insertion of the floppy.  (This

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               is the default.)  If N=0, do not prompt.  (Thus, this
               parameter is never needed.)

          'ramdisk_size=N' or (obsolete) 'ramdisk=N'
               Set the maximal size of the ramdisk(s) to N kB.  The
               default is 4096 (4 MB).

               Sets the starting block number (the offset on the
               floppy where the ramdisk starts) to N.  This is needed
               in case the ramdisk follows a kernel image.

               (Only if the kernel was compiled with
               CONFIG_BLK_DEV_RAM and CONFIG_BLK_DEV_INITRD.)  These
               days it is possible to compile the kernel to use ini-
               trd.  When this feature is enabled, the boot process
               will load the kernel and an initial ramdisk; then the
               kernel converts initrd into a "normal" ramdisk, which
               is mounted read-write as root device; then /linuxrc is
               executed; afterward the "real" root filesystem is
               mounted, and the initrd filesystem is moved over to
               /initrd; finally the usual boot sequence (e.g., invoca-
               tion of /sbin/init) is performed.

               For a detailed description of the initrd feature, see
               the kernel source file
               Documentation/admin-guide/initrd.rst (or
               Documentation/initrd.txt before Linux 4.10).

               The 'noinitrd' option tells the kernel that although it
               was compiled for operation with initrd, it should not
               go through the above steps, but leave the initrd data
               under /dev/initrd. (This device can be used only once:
               the data is freed as soon as the last process that used
               it has closed /dev/initrd.)

        Boot arguments for SCSI devices
          General notation for this section:

          iobase -- the first I/O port that the SCSI host occupies.
          These are specified in hexadecimal notation, and usually lie
          in the range from 0x200 to 0x3ff.

          irq -- the hardware interrupt that the card is configured to
          use.  Valid values will be dependent on the card in ques-
          tion, but will usually be 5, 7, 9, 10, 11, 12, and 15.  The
          other values are usually used for common peripherals like
          IDE hard disks, floppies, serial ports, and so on.

          scsi-id -- the ID that the host adapter uses to identify
          itself on the SCSI bus.  Only some host adapters allow you

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          to change this value, as most have it permanently specified
          internally.  The usual default value is 7, but the Seagate
          and Future Domain TMC-950 boards use 6.

          parity -- whether the SCSI host adapter expects the attached
          devices to supply a parity value with all information
          exchanges.  Specifying a one indicates parity checking is
          enabled, and a zero disables parity checking.  Again, not
          all adapters will support selection of parity behavior as a
          boot argument.

               A SCSI device can have a number of 'subdevices' con-
               tained within itself.  The most common example is one
               of the new SCSI CD-ROMs that handle more than one disk
               at a time.  Each CD is addressed as a 'Logical Unit
               Number' (LUN) of that particular device.  But most
               devices, such as hard disks, tape drives and such are
               only one device, and will be assigned to LUN zero.

               Some poorly designed SCSI devices cannot handle being
               probed for LUNs not equal to zero.  Therefore, if the
               compile-time flag CONFIG_SCSI_MULTI_LUN is not set,
               newer kernels will by default probe only LUN zero.

               To specify the number of probed LUNs at boot, one
               enters 'max_scsi_luns=n' as a boot arg, where n is a
               number between one and eight.  To avoid problems as
               described above, one would use n=1 to avoid upsetting
               such broken devices.

          SCSI tape configuration
               Some boot time configuration of the SCSI tape driver
               can be achieved by using the following:


               The first two numbers are specified in units of kB.
               The default buf_size is 32k B, and the maximum size
               that can be specified is a ridiculous 16384 kB.  The
               write_threshold is the value at which the buffer is
               committed to tape, with a default value of 30 kB.  The
               maximum number of buffers varies with the number of
               drives detected, and has a default of two.  An example
               usage would be:


               Full details can be found in the file
               Documentation/scsi/st.txt (or drivers/scsi/
               for older kernels) in the Linux kernel source.

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        Hard disks
          IDE Disk/CD-ROM Driver Parameters
               The IDE driver accepts a number of parameters, which
               range from disk geometry specifications, to support for
               broken controller chips.  Drive-specific options are
               specified by using 'hdX=' with X in 'a'en'h'.

               Non-drive-specific options are specified with the pre-
               fix 'hd='.  Note that using a drive-specific prefix for
               a non-drive-specific option will still work, and the
               option will just be applied as expected.

               Also note that 'hd=' can be used to refer to the next
               unspecified drive in the (a, ..., h) sequence.  For the
               following discussions, the 'hd=' option will be cited
               for brevity.  See the file Documentation/ide/ide.txt
               (or Documentation/ide.txt in older kernels, or
               drivers/block/README.ide in ancient kernels) in the
               Linux kernel source for more details.

          The 'hd=cyls,heads,sects[,wpcom[,irq]]' options
               These options are used to specify the physical geometry
               of the disk.  Only the first three values are required.
               The cylinder/head/sectors values will be those used by
               fdisk.  The write precompensation value is ignored for
               IDE disks.  The IRQ value specified will be the IRQ
               used for the interface that the drive resides on, and
               is not really a drive-specific parameter.

          The 'hd=serialize' option
               The dual IDE interface CMD-640 chip is broken as
               designed such that when drives on the secondary inter-
               face are used at the same time as drives on the primary
               interface, it will corrupt your data.  Using this
               option tells the driver to make sure that both inter-
               faces are never used at the same time.

          The 'hd=noprobe' option
               Do not probe for this drive.  For example,

                   hdb=noprobe hdb=1166,7,17

               would disable the probe, but still specify the drive
               geometry so that it would be registered as a valid
               block device, and hence usable.

          The 'hd=nowerr' option
               Some drives apparently have the WRERR_STAT bit stuck on
               permanently.  This enables a work-around for these bro-
               ken devices.

          The 'hd=cdrom' option

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               This tells the IDE driver that there is an ATAPI com-
               patible CD-ROM attached in place of a normal IDE hard
               disk.  In most cases the CD-ROM is identified automati-
               cally, but if it isn't then this may help.

          Standard ST-506 Disk Driver Options ('hd=')
               The standard disk driver can accept geometry arguments
               for the disks similar to the IDE driver.  Note however
               that it expects only three values (C/H/S); any more or
               any less and it will silently ignore you.  Also, it
               accepts only 'hd=' as an argument, that is, 'hda=' and
               so on are not valid here.  The format is as follows:


               If there are two disks installed, the above is repeated
               with the geometry parameters of the second disk.

        Ethernet devices
          Different drivers make use of different parameters, but they
          all at least share having an IRQ, an I/O port base value,
          and a name.  In its most generic form, it looks something
          like this:


          The first nonnumeric argument is taken as the name.  The
          param_n values (if applicable) usually have different mean-
          ings for each different card/driver.  Typical param_n values
          are used to specify things like shared memory address,
          interface selection, DMA channel and the like.

          The most common use of this parameter is to force probing
          for a second ethercard, as the default is to probe only for
          one.  This can be accomplished with a simple:


          Note that the values of zero for the IRQ and I/O base in the
          above example tell the driver(s) to autoprobe.

          The Ethernet-HowTo has extensive documentation on using mul-
          tiple cards and on the card/driver-specific implementation
          of the param_n values where used.  Interested readers should
          refer to the section in that document on their particular

        The floppy disk driver
          There are many floppy driver options, and they are all
          listed in Documentation/blockdev/floppy.txt (or
          Documentation/floppy.txt in older kernels, or
          drivers/block/README.fd for ancient kernels) in the Linux

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          kernel source.  See that file for the details.

        The sound driver
          The sound driver can also accept boot arguments to override
          the compiled-in values.  This is not recommended, as it is
          rather complex.  It is described in the Linux kernel source
          file Documentation/sound/oss/README.OSS
          (drivers/sound/Readme.linux in older kernel versions).  It
          accepts a boot argument of the form:


          where each deviceN value is of the following format 0xTaaaId
          and the bytes are used as follows:

          T - device type: 1=FM, 2=SB, 3=PAS, 4=GUS, 5=MPU401, 6=SB16,

          aaa - I/O address in hex.

          I - interrupt line in hex (i.e., 10=a, 11=b, ...)

          d - DMA channel.

          As you can see, it gets pretty messy, and you are better off
          to compile in your own personal values as recommended.
          Using a boot argument of 'sound=0' will disable the sound
          driver entirely.

        The line printer driver


               You can tell the printer driver what ports to use and
               what ports not to use.  The latter comes in handy if
               you don't want the printer driver to claim all avail-
               able parallel ports, so that other drivers (e.g., PLIP,
               PPA) can use them instead.

               The format of the argument is multiple port names.  For
               example, lp=none,parport0 would use the first parallel
               port for lp1, and disable lp0.  To disable the printer
               driver entirely, one can use lp=0.

          klogd(8), mount(8)

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          For up-to-date information, see the kernel source file
          Documentation/admin-guide/kernel-parameters.txt.  .}f

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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