C.4 Boot Parameters

Boot parameters are specified using a three-part directive that includes the name of the parameter and an optional list of options, which consists of an equal sign (=) followed by a comma-separated list of option values

No spaces may appear in the directive. As an example, the following directive specifies the identity of the Linux root partition:


The installation program generally refers to partitions by using labels, so that you can boot a system even if you move the partitions around. A root directive referring to a label looks like this:


You can specify multiple directives by separating them with a space. For example, the following specifies the identity of the Linux root partition and that the root partition is initially mounted read-only, so that a thorough check of its filesystem can be performed:

root=/dev/hda1 ro

Most directives are interpreted by the kernel, though GRUB is also capable of processing directives. If you specify a directive that neither the kernel nor GRUB understands (assuming you're using GRUB), a directive that includes an equal sign is passed to the init process as an environment variable. You learned about environment variables in Chapter 7. A nonkernel directive that doesn't include an equal sign is passed to the init process. An example of this usage is specifying the directive single, which causes init to start your system in single-user mode:

root=/dev/hda1 ro single

C.4.1 General Boot Arguments

Table C-2 describes some of the most popular and useful boot arguments. These arguments apply to your system as a whole; in subsequent sections, you'll learn about other boot arguments that apply to specific devices or functions. In addition to boot arguments previously introduced, the table describes the reserve argument, which is helpful in avoiding system memory conflicts.

Table C-2. Selected general boot arguments


Description and options


Specifies arguments passed by the kernel to the init process.


Specifies the amount of physical memory available to Linux; lets you instruct Linux to avoid high memory areas used by some systems for BIOS or caching. You can specify the amount as a hexadecimal number or as a decimal number followed by k or M, denoting kilobytes or megabytes, respectively.


Specifies I/O ports that must not be probed. The port number is specified by using a hexadecimal number, and the range is specified by using a decimal number. For example, reserve=0x320,32 specifies that I/O ports 320-33f must not be probed.


Initially mounts the root filesystem in read-only mode, so that a more effective filesystem check can be done.


Identifies the root filesystem:


Floppy disk n (0 or 1)


Partition n of IDE drive x (a to d)


Partition n of SCSI drive x (a to e)


Initially mounts the root filesystem in read/write mode; does not perform a filesystem check.


Specifies the default display mode set before booting. Specifying vga=ask will cause lilo to list the available video modes. You can then specify the desired mode in place of ask. (This argument is interpreted by lilo and will have no effect if another loader is used.)

C.4.2 RAM Disk Boot Arguments

Table C-3 describes four boot arguments used in working with RAM disks. You won't likely need to specify any of these, but knowing about them may help you understand boot specifications written by others, including those used by Red Hat Linux.

Table C-3. Selected RAM disk arguments


Description and options


Specifies that a RAM disk is not to be loaded (0) or is to be loaded (1).


Specifies whether to provide a prompt instructing the user to insert a floppy containing a RAM disk (1) or provide no such prompt (0).


Specifies the amount of RAM to be allocated to a RAM disk. If not specified, the default is 4 MB.


Specifies the offset (in disk blocks from the start of the boot media) of the RAM disk data; lets a kernel and RAM disk data occupy the same floppy disk.

C.4.3 SCSI Host Adapter Boot Arguments

Table C-4 describes the most often used boot arguments related to SCSI host adapters. Table C-5 describes the options used by the SCSI host adapter boot arguments and other boot arguments.

Table C-4. Selected SCSI host adapter arguments


Description and options


Advansys SCSI host adapter:


Adaptec aha151x, ada152x, aic6260, aic6360, and SB16-SCSI SCSI host adapters:


Adaptec aha154x SCSI host adapter:


Adaptec aha274x, aha284x, aic7xxx SCSI host adapters:


AMD AM53C974-based SCSI host adapters:


See the file linux/drivers/scsi/README.AM53C974.


BusLogic SCSI controller. Many options are available. See the BootPrompt-HOWTO.


EATA SCSI host adapter:


Future Domain SCSI controller:


Always SCSI host adapter. The driver for the Always SCSI controller accepts options in somewhat different format than other drivers. See the Boot Prompt-HOWTO.


Specifies the maximum number of SCSI logical units to be probed; lets you avoid probing devices that might lock up the SCSI bus.


NCR 5380-based SCSI host adapters:


NCR 53c400-based SCSI host adapters:


NCR 53c406a-based SCSI host adapters:


Pro Audio Spectrum SCSI host adapter:


Iomega parallel port SCSI adapter for ZIP drive:


Seagate ST-0x SCSI host adapter:


Trantor T128 SCSI host adapter:


Future Domain TMC-8xx and TMC-950 SCSI host adapters:


Ultrastor SCSI host adapter:


Western Digital WD7000 SCSI host adapter:


For example, from Table C-4 you can learn that Adaptec aha154x SCSI host adapters use a boot argument having the form:


Table C-5 helps you understand the form of the iobase option and the other italicized options. The iobase option, for example, lets you specify the I/O port associated with the SCSI host adapter. For example, you can specify a boot argument for an Adaptec aha154x SCSI host adapter by writing only an iobase option; the remaining options are optional. However, as indicated by the square brackets, if you include a buson option, you must include a busoff option. Similarly, to include the dmaspeed option, you must include each of the other options. Here's an example of a complete boot argument:


Table C-5. Selected boot prompt options


Description and options


The interval (number of microseconds) during which the device will relinquish the ISA bus, specified as a decimal integer, for example, 4.


The interval (number of microseconds) during which the device will dominate the ISA bus, specified as a decimal integer, for example, 11.


The I/O port used for control, specified as a hexadecimal number, for example, 0x300.


The geometry of the storage device, specified as three integers denoting the number of cylinders, heads, and sectors, respectively.


A SCSI device with which the host adapter communicates, specified as a decimal integer, for example, 2.


The DMA (direct memory access) channel by used by the device, specified as a decimal integer, for example, 3.


The rate (in MB/sec.) at which DMA transfers are performed, specified as a decimal integer, for example, 5.


Whether extended translation for large disks is enabled (1) or not (0).


Whether the driver attempts to work, even if the firmware version is unknown (79); other values are ignored.


Whether the driver should reset the SCSI bus when setting up the host adapter at boot (1) or not (0).


An I/O port, specified as a hexadecimal number, for example, 0x300.


A hardware interrupt number, specified as a decimal integer, for example, 5.


Whether a Pro Audio Spectrum (PAS) card is used; otherwise, do not specify this option.


The base address of a memory region used for memory-mapped I/ O, specified as a hexadecimal number, for example, 0x2000.


Whether the SCSI host adapter uses parity (1) or does not use parity (0).


Whether insl and outsl multibyte instructions (1) or inb and outb single-byte instructions (0) are used.


Whether the SCSI host adapter is allowed to disconnect and reconnect (1) or holds a connection until the operation is complete (0).


The ID by which the SCSI host adapter identifies itself, specified as a decimal integer, for example, 7.

To determine a proper value for options described in Table C-5, you must often know something about the hardware structure of your system. The procedures described in Chapter 2 will help you.

C.4.4 IDE Hard Drive and CD-ROM Boot Arguments

Table C-6 describes the most commonly used boot arguments associated with IDE hard drives and CD-ROM drives. Refer to Table C-5 to determine the form of the italicized options.

Table C-6. Selected IDE hard drive arguments


Description and options


IDE hard drive or CD-ROM (x denotes the physical device and must be a letter from a to h):


Specifies that the driver should attempt to tune the interface to the fastest possible mode and speed


Specifies that the drive is a CD-ROM drive


Specifies the geometry of the drive


Specifies that the drive is not present?do not probe


Specifies that the driver should not probe for the device


Specifies that the WRERR_STAT bit should be ignored on this drive


IDE hard drive or CD-ROM:


Probe for and support the alil4xx interface


Probe for and support the cmd640 chip (required for controllers using a VLB interface)


Probe for and support the dtc2278 interface


Probe for and support the ht6560b interface


Probe for and support the qd6580 interface


Probe for and support the umc8672 interface


IDE hard drive or CD-ROM (x specifies the physical device and must be a digit from 0 to 3):


Specifies that the driver should attempt to tune the interface to the fastest possible mode and speed


Specifies the I/O port used by the drive


Specifies the I/O port and control port used by the drive


Specifies the I/O port, control port, and IRQ used by the drive


Specifies that the driver should not attempt to tune the interface for fastest mode and speed


Specifies that the driver should not probe for the device


Specifies that I/O operations should not be overlapped

C.4.5 Non-IDE CD-ROM Drive Boot Arguments

Table C-7 describes the most common boot arguments for non-IDE CD-ROM drives. Refer to Table C-5 to determine the form of the italicized options.

Table C-7. Selected CD-ROM arguments


Description and options


Aztech CD-ROM:


Sony CDU-31A or CDU-33A CD-ROM:


Sony CDU-535 CD-ROM:


Goldstar CD-ROM:




Mitsumi CD-ROM:


Optical Storage CD-ROM:


Phillips CD206 CD-ROM:


Sanyo CD-ROM:


SoundBlaster Pro CD-ROM:


[C] Don't include the square brackets in your boot argument; they merely indicate which options must be present.

C.4.6 Floppy Drive Boot Arguments

A few systems require special boot arguments to best use their floppy drives. Table C-8 describes the most common boot arguments related to floppy drives. Floppy drives that are not well behaved may malfunction if you specify the daring option, which you should use only with care. For additional boot arguments related to floppy drives, see /usr/src/linux/Documentation/floppy.txt.

Table C-8. Selected floppy disk arguments and options

Argument and option



Specifies that only units 0 and 1 are allowed, to work around problem with BIOS of certain ASUS motherboards.


Specifies that the floppy controller is well behaved, allowing more efficient operation.


Specifies that the floppy controller may not be well behaved (default).


Specifies that the system is an IBM ThinkPad.

floppy=no_unexpected_ interrupts or floppy=L40SX

Specifies that a message should be printed when an unexpected interrupt is received. This is required by IBM L40SX laptops in certain video modes.

C.4.7 Bus Mouse Boot Arguments

Two boot arguments provide bus mouse support. The first supports the Microsoft bus mouse:


The second supports any non-Microsoft bus mouse:


Each argument accepts a single option specifying the IRQ associated with the mouse.

C.4.8 Parallel Port Printer Boot Arguments

The Linux printer driver claims all available parallel ports. If you want to access a device other than a printer attached to a parallel port, you must instruct the printer driver to reserve only the ports associated with printers. To do so, use the lp boot argument, which takes as its options a list of ports and IRQs used to support printers. For example, the following boot argument specifies two printers:


The first printer is on port 0x3bc and the second is on port 0x378. The first printer uses a special IRQ-less mode known as polling, so its IRQ is specified as 0. The second printer uses IRQ 7.

To disable all printers, specify lp=0.

C.4.9 Loadable Ethernet Drivers

Early versions of Linux used a so-called monolithic kernel. At that time, Linux distributions typically included several kernels, offering support for a variety of devices that might be needed to boot and install a Linux system. Devices not needed to boot and install a system?so-called special devices?had second-class status. To access special devices, users had to compile customized kernels that included support for those devices. When adding a device to a system, users often had to compile a new kernel, which was something of an inconvenience.

More recent versions of Linux feature a modular kernel, which allows drivers to be dynamically loaded on command. This makes it much easier than before to configure your Linux system to support Ethernet cards and other special devices. Red Hat Linux is generally able to configure your primary Ethernet card automatically, by probing for it during installation of Linux.

However, the autoprobe doesn't always succeed. Moreover, if you have more than one Ethernet card, the installation program sets up only the first card it finds. To set up additional cards, you need to know a bit about Linux's loadable modules.

C.4.10 Dynamically Loading a Modular Driver

To dynamically load a modular driver, issue the following command:

#  modprobe  driver 

where driver specifies the module to be loaded. As an example, the command:

#  modprobe ne2k-pci 

loads the modular driver for the PCI-based NE2000 Ethernet card.

To find out what network adapters are supported by Red Hat Linux or to find out what driver to use with a particular adapter, see the Red Hat Linux Hardware Compatibility List, http://hardware.redhat.com.

When a driver is loaded, it generally probes to locate the supported device. In case an autoprobe fails, most drivers let you specify the I/O port and IRQ by using a command like the following:

# modprobe ne2k=pci io=0x280 irq=11

Some cards support additional options; these are documented in the file /usr/src/ linux/Documentation/networking/net-modules.txt.

C.4.11 Loading Modular Drivers at Boot Time

The Linux kernel automatically loads modules specified in the module configuration file, /etc/modules.conf. So, once you've determined the proper module and options required by your Ethernet card, you can add a line or two to the module configuration file so that your card will be made ready to operate each time you boot your system.

The alias directive associates a logical module name with an actual module. Logical module names specify types of devices; for example, eth0 specifies the first Ethernet card in a system, and eth1 specifies the second Ethernet card in a system. Suppose your system includes two Ethernet cards: a non-PCI-based NE2000 and an SMC EtherPower, which is based on DEC's TULIP chip. You could use the following directives to automatically load these modules at boot time:

alias eth0 ne
alias eth1 tulip

If a driver requires options, you can specify them by using an options directive, which has the following form:

options driver argument=value[,value,...] argument=value[,value,...] ...

For example, you might specify the I/O port and IRQ used by the NE2000 card like this:

options ne io=0x280 irq=12

Most ISA modules accept parameters like io=0x340 and irq=12 on the insmod command line. You should supply these parameters to avoid probing for the card. Unlike PCI and EISA devices, ISA devices sometimes cannot be safely autoprobed.

C.4.12 Administering Modular Drivers

The lsmod command, which takes no arguments, lists the loaded modular drivers. To unload a modular driver, specify the driver as the argument of the modprobe command and specify the -r argument. For example, to remove the ne driver, issue the command:

# modprobe -r ne

To unload every unused module?that is, every module not associated with an operational device?invoke the rmmod command and specify the -a argument:

# rmmod -a

You can't remove a module that's in use; therefore, you must shut down the device before removing it. To shut down an Ethernet device, you can use Neat. Or you can issue the following command:

# ifconfig eth n  down 

where ethn specifies the logical device (for example, eth0 or eth1).