− introduction to boot time parameters of 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 kernel command line is parsed into a list of strings
(boot arguments) separated by spaces. Most of the boot
arguments have the form:
’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
’nfsaddrs=’, ’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 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
Anything of the
form ’foo=bar’ that is not accepted as a setup
function as described above is then interpreted as an
environment variable to be set. A (useless?) example would
be to use ’TERM=vt100’ as a boot argument.
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.
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
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 ’root=/dev/fd1’.
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
compatible disk, with Y in ’a’-’e’),
Y the driver letter or number, and N the number (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
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
This parameter sets the delay
(in seconds) to pause before attempting to mount the root
This parameter sets the mount
option string for the root filesystem (see also
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.
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 mount(8).)
’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:
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 want the kernel to
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.
the boot line
device drivers except the driver for ’blah’ from
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 >
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
(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 number of CPUs activated in SMP mode to N.
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 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
(described in syslog(2)), the syslog(2)
SYSLOG_ACTION_CONSOLE_LEVEL operation, or
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
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.
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.
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/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.)
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
(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)
For a detailed
description of the initrd feature, see the kernel source
’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
arguments for SCSI devices
General notation for this section:
-- 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.
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.
-- 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 Future Domain TMC-950 boards use 6.
-- 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
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 assigned to LUN
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
Some boot time configuration of
the SCSI tape driver can be achieved by using the
The first two
numbers are specified in units of kB. The default
buf_size is 32kB, and the maximum size that can be
specified is a ridiculous 16384kB. The
write_threshold is the value at which the buffer is
committed to tape, with a default value of 30kB. The maximum
number of buffers varies with the number of drives detected,
and has a default of two. An example usage would be:
can be found in the file Documentation/scsi/st.txt
(or drivers/scsi/README.st for older kernels) in the
Linux kernel source.
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
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
following discussions, the ’hd=’ option will be
cited for brevity. See the file
Documentation/ide.txt in older kernels, or
drivers/block/README.ide in ancient kernels) in the
Linux kernel source for more details.
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 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.
Do not probe for this drive.
the probe, but still specify the drive geometry so that it
would be registered as a valid block device, and hence
Some drives apparently have the
WRERR_STAT bit stuck on permanently. This enables a
work-around for these broken devices.
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
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.
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
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
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.
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
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 kernel source. See that file for the details.
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
(drivers/sound/Readme.linux in older kernel
versions). It accepts a boot argument of the form:
deviceN value is of the following format 0xTaaaId and the
bytes are used as follows:
device type: 1=FM, 2=SB, 3=PAS, 4=GUS, 5=MPU401, 6=SB16,
aaa − I/O
address in hex.
interrupt line in hex (i.e 10=a, 11=b, ...)
d − DMA
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
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.
information, see the kernel source file
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