CONTENTS

DESCRIPTION

When the kernel is booted directly by the BIOS (say from a floppy to which you copied a kernel using ‘cp zImage /dev/fd0’), you have no opportunity to specify any parameters. So, in order to take advantage of this possibility you have to use software that is able to pass parameters, like LILO or loadlin. For a few parameters one can also modify the kernel image itself, using rdev, see rdev(8) for further details.

The LILO program (LInux LOader) written by Werner Almesberger is the most commonly used. It has the ability to boot various kernels, and stores the configuration information in a plain text file. (See lilo(8) and lilo.conf(5).) LILO can boot DOS, OS/2, Linux, FreeBSD, UnixWare, etc., and is quite flexible.

The other commonly used Linux loader is ‘LoadLin’ which is a DOS program that has the capability to launch a Linux kernel from the DOS prompt (with boot-args) assuming that certain resources are available. This is good for people that want to launch Linux from DOS.

It is also very useful if you have certain hardware which relies on the supplied DOS driver to put the hardware into a known state. A common example is ‘SoundBlaster Compatible’ sound cards that require the DOS driver to twiddle a few mystical registers to put the card into a SB compatible mode. Booting DOS with the supplied driver, and then loading Linux from the DOS prompt with loadlin avoids the reset of the card that happens if one rebooted instead.

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

Most of the sorting goes on in linux/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 arguments is described below.

Then it walks a list of setup functions (contained in the bootsetups array) 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 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 kernel and were not interpreted as environment variables are then passed onto process one, which is usually the init program. The most common argument that is passed to the init process is the word ‘single’ which instructs init to boot the computer in single user mode, and not launch all the usual daemons. Check the manual page for the version of init installed on your system to see what arguments it accepts.

This sets the initial command to be executed by the kernel. If this is not set, or cannot be found, the kernel will try /etc/init, then /bin/init, then /sbin/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.

(Only when CONFIG_BUGi386 is defined.) Some i387 coprocessor chips have bugs that show up when used in 32 bit protected mode. For example, some of the early ULSI-387 chips would cause solid lockups while performing floating point calculations. Using the ‘no387’ boot arg causes Linux to ignore the maths coprocessor even if you have one. Of course you must then have your kernel compiled with math emulation support!

(Only when CONFIG_BUGi386 is defined.) Some of the early i486DX-100 chips have a problem with the ‘hlt’ instruction, in that they can’t reliably return to operating mode after this instruction is used. Using the ‘no-hlt’ instruction tells Linux to just run an infinite loop when there is nothing else to do, and to not halt the CPU. This allows people with these broken chips to use Linux.

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 also be set using rdev(8).)

The root device can be specified symbolically or numerically. A symbolic specification has the form /dev/XXYN, where XX designates the device type (‘hd’ for ST-506 compatible hard disk, with Y in ‘a’-‘d’; ‘sd’ for SCSI compatible disk, with Y in ‘a’-‘e’; ‘ad’ for Atari ACSI disk, with Y in ‘a’-‘e’, ‘ez’ for a Syquest EZ135 parallel port removable drive, with Y=‘a’, ‘xd’ for XT compatible disk, with Y either ‘a’ or ‘b’; ‘fd’ for floppy disk, with Y the floppy drive number - fd0 would be the DOS ‘A:’ drive, and fd1 would be ‘B:’), Y the driver letter or number, and N the number (in decimal) of the partition on this device (absent in the case of floppies). Recent kernels allow many other types, mostly for CD-ROMs: nfs, ram, scd, mcd, cdu535, aztcd, cm206cd, gscd, sbpcd, sonycd, bpcd. (The type nfs specifies a net boot; ram refers to a ram disk.)

Note that this has nothing to do with the designation of these devices on your file system. 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. (E.g., /dev/sda3 is major 8, minor 3, so you could use ‘root=0x803’ as an alternative.)

The ‘ro’ option tells the kernel to mount the root filesystem as ‘readonly’ so that filesystem consistency check programs (fsck) can do their work on a quiescent file system. No processes can write to files on the filesystem in question until it is ‘remounted’ as read/write capable, e.g., 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.

The choice between read-only and read/write can also be set using rdev(8).

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

The BIOS call defined in the PC specification that returns the amount of installed memory was only designed to be able to report up to 64MB. Linux uses this BIOS call at boot to determine how much memory is installed. If you have more than 64MB of RAM installed, you can use this boot arg to tell Linux how much memory you have. The value is in decimal or hexadecimal (prefix 0x), and the suffixes ‘k’ (times 1024) or ‘M’ (times 1048576) can be used. Here is a quote from Linus on usage of the ‘mem=’ parameter.

‘‘The kernel will accept any ‘mem=xx’ parameter you give it, and if it turns out that you lied to it, it will crash horribly sooner or later. The parameter indicates the highest addressable RAM address, so ‘mem=0x1000000’ means you have 16MB of memory, for example. For a 96MB machine this would be ‘mem=0x6000000’.

NOTE NOTE NOTE: some machines might use the top of memory for BIOS cacheing or whatever, so you might not actually have up to the full 96MB addressable. The reverse is also true: some chipsets will map the physical memory that is covered by the BIOS area into the area just past the top of memory, so the top-of-mem might actually be 96MB + 384kB for example. If you tell linux that it has more memory than it actually does have, bad things will happen: maybe not at once, but surely eventually.’’

You can also use the boot argument ‘mem=nopentium’ to turn off 4 MB pagetables on kernels configured for IA32 systems with a pentium or newer CPU.

Kernel messages are handed off to the kernel log daemon klogd so that they may be logged to disk. Messages with a priority above console_loglevel are also printed on the console. (For these levels, see <linux/kernel.h>.) By default this variable is set to log anything more important than debug messages. This boot argument will cause the kernel to also print the messages of DEBUG priority. The console loglevel can also be set at run time via an option to klogd. See klogd(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:

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, or by use of rdev(8).) These days ram disks use the buffer cache, and grow dynamically. For a lot of information (e.g., how to use rdev(8) in conjunction with the new ramdisk setup), see /usr/src/linux/Documentation/ramdisk.txt.

There are four parameters, two boolean and two integral.

For a detailed description of the initrd feature, see /usr/src/linux/Documentation/initrd.txt.

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.)

General notation for this section:

iobase -- the first I/O port that the SCSI host occupies. These are specified in hexidecimal 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, etc.

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 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 behaviour as a boot argument.

A SCSI device can have a number of ‘sub-devices’ 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 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 only probe 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.

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

The aha numbers refer to cards and the aic numbers refer to the actual SCSI chip on these type of cards, including the Soundblaster-16 SCSI.

The probe code for these SCSI hosts looks for an installed BIOS, and if none is present, the probe will not find your card. Then you will have to use a boot arg of the form:

The aha1542 series cards have an i82077 floppy controller onboard, while the aha1540 series cards do not. These are busmastering cards, and have parameters to set the ‘‘fairness’’ that is used to share the bus with other devices. The boot arg looks like the following.

The buson", "busoff values refer to the number of microseconds that the card dominates the ISA bus. The defaults are 11us on, and 4us off, so that other cards (such as an ISA LANCE Ethernet card) have a chance to get access to the ISA bus.

The dmaspeed value refers to the rate (in MB/s) at which the DMA (Direct Memory Access) transfers proceed. The default is 5MB/s. Newer revision cards allow you to select this value as part of the soft-configuration, older cards use jumpers. You can use values up to 10MB/s assuming that your motherboard is capable of handling it. Experiment with caution if using values over 5MB/s.

These boards can accept an argument of the form:

The AdvanSys driver can accept up to four i/o addresses that will be probed for an AdvanSys SCSI card. Note that these values (if used) do not effect EISA or PCI probing in any way. They are only used for probing ISA and VLB cards. In addition, if the driver has been compiled with debugging enabled, the level of debugging output can be set by adding an 0xdeb[0-f] parameter. The 0-f allows setting the level of the debugging messages to any of 16 levels of verbosity.

The parameters N1-N5 are integers. The parameters S1,... are strings. N1 is the I/O Address at which the Host Adapter is located. N2 is the Tagged Queue Depth to use for Target Devices that support Tagged Queuing. N3 is the Bus Settle Time in seconds. This is the amount of time to wait between a Host Adapter Hard Reset which initiates a SCSI Bus Reset and issuing any SCSI Commands. N4 is the Local Options (for one Host Adapter). N5 is the Global Options (for all Host Adapters).

The string options are used to provide control over Tagged Queuing (TQ:Default, TQ:Enable, TQ:Disable, TQ:<Per-Target-Spec>), over Error Recovery (ER:Default, ER:HardReset, ER:BusDeviceReset, ER:None, ER:<Per-Target-Spec>), and over Host Adapter Probing (NoProbe, NoProbeISA, NoSortPCI).

The PAS16 uses a NC5380 SCSI chip, and newer models support jumperless configuration. The boot arg is of the form:

If your card is not detected at boot time, you will then have to use a boot arg of the form:

These cards are also based on the NCR5380 chip, and accept the following options:

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 following discussions, the ‘hd=’ option will be cited for brevity. See the file README.ide in linux/drivers/block 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.

This option tells the driver that you have a DTC-2278D IDE interface. The driver then tries to do DTC specific operations to enable the second interface and to enable faster transfer modes.

Do not probe for this drive. For example,

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.

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

If you are unfortunate enough to be using one of these old 8 bit cards that move data at a whopping 125kB/s then here is the scoop. If the card is not recognised, you will have to use a boot arg of the form:

The syntax for this type of card is:

This CD-ROM interface is found on some of the Pro Audio Spectrum sound cards, and other Sony supplied interface cards. The syntax is as follows:

The is_pas_card should be entered as ‘PAS’ if using a Pro Audio Spectrum card, and otherwise it should not be specified at all.

The syntax for this CD-ROM interface is:

The syntax for this CD-ROM interface is:

The syntax for this CD-ROM interface is:

This is for the same hardware as above, but the driver has extended features. Syntax:

The syntax for this type of card is:

The syntax for this type of card is:

The driver assumes numbers between 3 and 11 are IRQ values, and numbers between 0x300 and 0x370 are I/O ports, so you can specify one, or both numbers, in any order. It also accepts ‘cm206=auto’ to enable autoprobing.

The syntax for this type of card is:

The syntax for this type of card is:

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 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.

There are many floppy driver options, and they are all listed in README.fd in linux/drivers/block. This information is taken directly from that file.

Sets the bitmask of allowed drives to mask. By default, only units 0 and 1 of each floppy controller are allowed. This is done because certain non-standard hardware (ASUS PCI motherboards) mess up the keyboard when accessing units 2 or 3. This option is somewhat obsoleted by the cmos option.

Sets the bitmask of allowed drives to all drives. Use this if you have more than two drives connected to a floppy controller.

Sets the bitmask to allow only units 0 and 1. (The default)

Tells the floppy driver that you have a well behaved floppy controller. This allows more efficient and smoother operation, but may fail on certain controllers. This may speed up certain operations.

Tells the floppy driver that your floppy controller should be used with caution.

Tells the floppy driver that you have only floppy controller (default)

Tells the floppy driver that you have two floppy controllers. The second floppy controller is assumed to be at address. If address is not given, 0x370 is assumed.

Tells the floppy driver that you have a Thinkpad. Thinkpads use an inverted convention for the disk change line.

Tells the floppy driver that you don’t have a Thinkpad.

Sets the cmos type of drive to type. Additionally, this drive is allowed in the bitmask. This is useful if you have more than two floppy drives (only two can be described in the physical cmos), or if your BIOS uses non-standard CMOS types. Setting the CMOS to 0 for the first two drives (default) makes the floppy driver read the physical cmos for those drives.

Print a warning message when an unexpected interrupt is received (default behaviour)

Don’t print a message when an unexpected interrupt is received. This is needed on IBM L40SX laptops in certain video modes. (There seems to be an interaction between video and floppy. The unexpected interrupts only affect performance, and can safely be ignored.)

The sound driver can also accept boot args to override the compiled in values. This is not recommended, as it is rather complex. It is described in the Readme.Linux file, in linux/drivers/sound. It accepts a boot arg of the form:

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 arg of ‘sound=0’ will disable the sound driver entirely.

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.

atamouse=threshold[,y-threshold]

"VIDEO HARDWARE"