bootparam(7)                                      Miscellaneous Information Manual                                     bootparam(7)

       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 ad‐
       vantage 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.   (How‐
       ever,  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=', 'nfsaddrs=', 'ro', 'rw', 'debug', or 'init'.  The meaning of these special ar‐
       guments is described below.

       Then it walks a list of setup functions to see if the specified 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 envi‐
       ronment 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 kernel and were not interpreted as  environment  variables  are  then
       passed onto PID 1, which is usually the init(1) program.  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 usually 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

              The root device can be specified symbolically or numerically.  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'–'d'; 'sd' for SCSI compati‐
              ble disk, with Y in 'a'–'e'), Y the driver letter or number, and N the number (in decimal) of the partition  on  this

              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 specification of the above possible root devices in major/minor format  is
              also accepted.  (For example, /dev/sda3 is major 8, minor 3, so you could use 'root=0x803' as an alternative.)

              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 media.

       'ro' and 'rw'
              The  'ro'  option  tells  the kernel to mount the root filesystem as 'read-only' so that filesystem consistency check
              programs (fsck) can do their work on a quiescent 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

              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  hiber‐
              nation.  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 iden‐
              tified, or merely hardware you don't 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 argument 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 seconds  (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 re‐
              boot  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 BIOS.

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

   Boot arguments for use by kernel developers
              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 sys‐
              log(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  (described  in  syslog(2)),  the  syslog(2)  SYSLOG_ACTION_CON‐
              SOLE_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 specify‐
              ing  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  readpro‐
              file.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 drastically.  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  current  ramdisk  setup,
              see the kernel source file Documentation/blockdev/ramdisk.txt (Documentation/ramdisk.txt in older kernels).

              There are four parameters, two boolean and two integral.

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

              If N=1, do prompt for insertion of the floppy.  (This 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 initrd.  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., invocation 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 question,
       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 to
       change this value, as most have it permanently specified internally.  The usual default value is 7, but the Seagate and  Fu‐
       ture Domain TMC-950 boards use 6.

       parity  --  whether  the  SCSI  host  adapter expects the attached devices to supply a parity value with all information ex‐
       changes.  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' contained 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 as‐
              signed 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.

   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'–'h'.

              Non-drive-specific  options  are specified with the prefix '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 follow‐
              ing  discussions,  the 'hd=' option will be cited for brevity.  See the file Documentation/ide/ide.txt (or Documenta‐
              tion/ide.txt in older kernels, or drivers/block/README.ide in ancient kernels) in the Linux kernel  source  for  more

       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 interface  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 interfaces 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 broken de‐

       The 'hd=cdrom' option
              This tells the IDE driver that there is an ATAPI compatible CD-ROM attached in place of a normal IDE hard  disk.   In
              most cases the CD-ROM is identified automatically, 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 meanings  for
       each  different card/driver.  Typical param_n values are used to specify things like shared memory address, interface selec‐
       tion, 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 multiple 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 card.

   The floppy disk driver
       There are many floppy  driver  options,  and  they  are  all  listed  in  Documentation/blockdev/floppy.txt  (or  Documenta‐
       tion/floppy.txt in older kernels, or drivers/block/README.fd for ancient kernels) in the Linux 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  (driv‐
       ers/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, 7=SB16-MPU401

       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 available parallel ports, so that other drivers (e.g., PLIP, PPA) can  use  them

              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)

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

Linux man-pages 6.03                                         2023-02-05                                                bootparam(7)