bootparam(7)

✨ NAME ✨

bootparam — okay bestie, this is your intro to boot time parameters of the Linux kernel. iykyk 💖

this page lives in the Miscellaneous Information Manual, section 7, so like… bootparam(7). periodt.

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🌸 DESCRIPTION 🌸

so when the Linux kernel gets started, it accepts certain 'command-line options' or 'boot time parameters'. basically it's how you hand the kernel info it literally cannot figure out on its own, ngl — like hardware parameters it wouldn't have any way to determine by itself — or how you duck/override the values it would have detected. lowkey so powerful 💅

here's the thing though: when the kernel is booted directly by the BIOS, you get zero opportunity to specify any parameters. like, none. so to actually take advantage of this whole possibility, you gotta use a boot loader that's able to pass parameters along — something like GRUB. bestie, GRUB is doing the most for you fr fr 🫶

🎀 The argument list

the kernel command line gets parsed into a list of strings (these are the boot arguments), separated by spaces. most of the boot arguments have this form:

    name[=value_1][,value_2]...[,value_10]

where name is a unique keyword that identifies what part of the kernel the associated values (if there even are any) should be given to. and bestie, note this — the limit of 10 is real. like, genuinely real. the present code handles only 10 comma separated parameters per keyword. (but, okay, in unusually complicated situations you can reuse the same keyword with up to an additional 10 parameters, assuming the setup function supports it. iconic loophole energy 😌)

most of the sorting is coded in the kernel source file init/main.c. so here's the order of operations:

• first, the kernel checks whether the argument is any of these special arguments: 'root=', 'nfsroot=', 'nfsaddrs=', 'ro', 'rw', 'debug', or 'init'. the meaning of these special ones 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. so 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. and if it was? it would call the setup function associated with 'foo' (that's foo_setup()) and hand it the arguments 3, 4, 5, and 6 as given on the kernel command line. chef's kiss 🤌
• 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 PID 1, which is usually the init(1) program. the most common argument 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

• 'init=...' — this sets the initial command to be executed by the kernel. if this is not set, or it cannot be found, the kernel will try /sbin/init, then /etc/init, then /bin/init, then /bin/sh, and then panic if all of this fails 😭
• 'nfsaddrs=...' — sets the NFS boot address to the given string. this boot address is used in case of a net boot.
• 'nfsroot=...' — sets the NFS root name to the given string. if this string does not begin with / or , or a digit, then it gets prefixed by /tftpboot/. this root name is used in case of a net boot.
• 'root=...' — this one 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 it's 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 conventional. it's giving ✨tradition✨
  • 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.)
• 'rootdelay=' — this parameter sets the delay (in seconds) to pause before attempting to mount the root filesystem. a lil breather, as a treat 🧚
• 'rootflags=...' — sets the mount option string for the root filesystem (see also fstab(5)).
• 'rootfstype=...' — 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 needing to boot the box from alternate media. glow-down that we love 🔥
• 'ro', 'rw' — okay so:
  • 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).)
  • the 'rw' option tells the kernel to mount the root filesystem read/write. this is the default.
• 'resume=...' — tells the kernel the location of the suspend-to-disk data that you want the machine to resume from after hibernation. usually it's the same as your swap partition or file. example:

    resume=/dev/hda2

• 'reserve=...' — this is used to protect I/O port regions from probes. the form of the command is:

    reserve=iobase,extent[,iobase,extent]...

• on 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 want the kernel to initialize. all valid reasons, and honestly? relatable 💀
• 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. icon behavior honestly 👑

• 'panic=N' — 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

• 'reboot=[warm|cold][,[bios|hard]]' — 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 BIOS.
• 'nosmp', '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 number of CPUs activated in SMP mode to N.

💀 Boot arguments for use by kernel developers

• 'debug' — 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 dmesg(8). so many doors bestie 🚪✨
• 'profile=N' — it's 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.)

so in general, it's a bad idea to use a ramdisk under Linux — the system will use available memory more efficiently itself. no cap. 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. it's giving "prerequisites, but make it boot" 🎀

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.rst.

there are four parameters, two boolean and two integral. ✨

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and that's where part 1 stops bestie — those four ramdisk parameters are literally coming right up, don't go anywhere 💖

• load_ramdisk=N — if N=1, yes bestie, go ahead and load a ramdisk 💖. If N=0, do not load a ramdisk. (N=0 is the default, so that's just the vibe out of the box.)
• prompt_ramdisk=N — if N=1, do prompt for insertion of the floppy. (That's the default!) If N=0, do not prompt. Which means, and I cannot stress this enough, this parameter is never needed 😭. It literally cannot do anything new.
• ramdisk_size=N or (obsolete 🫠) ramdisk=N — set the maximal size of the ramdisk(s) to N kB. Default is 4096 (that's 4 MB for the girlies not doing math).
• ramdisk_start=N — sets the starting block number (aka the offset on the floppy where the ramdisk starts) to N. Bestie you need this when the ramdisk follows a kernel image, otherwise it's giving chaos.
• noinitrd — (only if the kernel was compiled with CONFIG_BLK_DEV_RAM and CONFIG_BLK_DEV_INITRD.) Okay so these days you can compile the kernel to use initrd. When that feature is enabled, the boot process loads the kernel and an initial ramdisk; then the kernel converts initrd into a "normal" ramdisk, which gets mounted read-write as the 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 (like, say, invocation of /sbin/init) happens. Iconic pipeline honestly ✨

For a detailed description of the initrd feature, go read the kernel source file Documentation/admin-guide/initrd.rst.

The noinitrd option tells the kernel: "I know you were compiled for operation with initrd, but don't do all that. Just leave the initrd data under /dev/initrd." Heads up though — this device can be used only once: the data is freed as soon as the last process that used it has closed /dev/initrd. One shot. Not a rewatch 💀

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🌈 Boot arguments for SCSI devices

General notation for this section, so we're all on the same page:

• 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 depend on the card in question, but will usually be 5, 7, 9, 10, 11, 12, and 15. The other values are usually already taken by common peripherals like IDE hard disks, floppies, serial ports, and so on. They're booked, sorry 💅
• scsi-id — the ID that the host adapter uses to identify itself on the SCSI bus. Only some host adapters let you change this value, because most have it permanently specified internally. The usual default value is 7, but the Seagate and Future Domain TMC-950 boards use 6. Not like other girls 🎀
• parity — whether the SCSI host adapter expects the attached devices to supply a parity value with all information exchanges. Specifying a one means parity checking is enabled, and a zero disables parity checking. Again, not all adapters will support selection of parity behavior as a boot argument. Fickle behavior fr fr.

max_scsi_luns=... 👑

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, like 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. Protect their peace, honestly 🫶

SCSI tape configuration

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

st=buf_size[,write_threshold[,max_bufs]]

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:

st=32,30,2

Full details can be found in the file Documentation/scsi/st.rst.

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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–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. Flexible, we love that ✨

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.

⚠️ Big one: support for IDE was removed in Linux 5.14. RIP to a real one 💀

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. Not a vibe, not a slay, just pain 😭. Using this option tells the driver to make sure that both interfaces are never used at the same time. It's a bodyguard, lowkey.

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. Still on the guest list, just not being searched for 💅

The hd=nowerr option

Some drives apparently have the WRERR_STAT bit stuck on permanently. This enables a work-around for these broken devices. Girl needs a support system, we get it 🫶

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:

hd=cyls,heads,sects

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

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🦋 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:

ether=irq,iobase[,param_1[,...param_8]],name

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 selection, DMA channel and the like. Very "it depends on your specific situation" energy 🌸

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:

ether=0,0,eth1

Note that the values of zero for the IRQ and I/O base in the above example tell the driver(s) to autoprobe. Let it figure itself out, periodt.

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. Go read, bestie 📚

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🧚 The floppy disk driver

There are many floppy driver options, and they are all listed in Documentation/blockdev/floppy.rst. See that file for the details. They're not gonna fit in this group chat 💖

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🔊 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:

sound=device1[,device2[,device3...[,device10]]]

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. Silence, but make it intentional.

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🖨️ The line printer driver

lp=

Syntax:

lp=0
lp=auto
lp=reset
lp=port[,port...]

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 instead. Sharing is caring, bestie 💜

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.

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👀 SEE ALSO 👀

• klogd(8)
• mount(8)

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

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And that's the whole thing, bestie, we made it to the end alive 🌈✨ This was bootparam(7) from Linux man-pages 6.20, dated 2026-02-08 — go forth and boot responsibly, mwah 💅🎀