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1. Introduction

The kernel has the capability to accept information at boot in the form of a `command line', similar to an argument list you would give to a program. 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.

It is the job of the boot loader (e.g. LILO, loadlin or Grub) to take this information from the user and put it in a previously agreed upon place where the kernel can find it once it starts.

This present revision covers kernels up to and including v2.4.20. and v2.5.63

The BootPrompt-Howto is by:

Paul Gortmaker, p_gortmaker @ yahoo.com

This document is Copyright (c) 1995-2003 by Paul Gortmaker. Please see the Disclaimer and Copying information at the end of this document ( copyright) for information about redistribution of this document and the usual `we are not responsible for what you manage to break...' type legal stuff.

1.1 Intended Audience and Applicability

Most Linux users should never have to even look at this document. Linux does an exceptionally good job at detecting most hardware and picking reasonable default settings for most parameters. The information in this document is aimed at users who might want to change some of the default settings to optimize the kernel to their particular machine, or to a user who has `rolled their own' kernel to support a not so common piece of hardware for which the automatic defaults are not optimal.

For the sake of this document it is best to break the boot arguments into two general categories; (a)ones handled by the kernel and (b)those being handled by a device driver. Examples would be init= which tells the kernel what the first program to run should be, versus aha154x= which tells a device driver for a SCSI card what hardware resources it should use are. This document concentrates on giving detailed information on those in (a) for reasons outlined below.

IMPORTANT NOTE: Driver related boot prompt arguments only apply to hardware drivers that are compiled directly into the kernel. They have no effect on drivers that are loaded as modules. Most Linux distributions come with a basic `bare-bones' kernel, and the drivers are small modules that are loaded after the kernel has initialized. If you are unsure if you are using modules then try lsmod, look at man depmod and man modprobe along with the contents of your /etc/modules.conf.

In light of this, device driver boot prompt arguments are only really used by a few people who are building their own kernels, and thus have the kernel source at hand. These people are usually going to check the source for the options and syntax required by that driver to get the most up to date info.

For example, if you were looking for what arguments could be passed to the AHA1542 SCSI driver, then you would go to the linux/drivers/scsi directory, and look in the file aha1542.c for __setup(... , ...). The first thing in brackets is the argument you provide at boot, and the second thing is the name of the function that processes your argument. Usually near the top of this function or at the top of the source file you will find a description of the boot time arguments that the driver accepts.

1.2 Related Documentation

For a while now, the kernel source has come with the file linux/Documentation/kernel-parameters.txt. This file contains a brief listing of all the boot time arguments that you can provide, along with quick pointers to where in the source you can find where the arguments are parsed. The idea is that this file gives developers a quick and easy place to add in a brief description of any new arguments that they add while working on the source. As such, it will probably always be more up to date than this document. Actually, I'm considering discontinuing this document in light of the existence of kernel-parameters.txt. (Opinions?)

The linux directory is usually found in /usr/src/ for most distributions. All references in this document to files that come with the kernel will have their pathname abbreviated to start with linux - you will have to add the /usr/src/ or whatever is appropriate for your system. Some distributions may not install the full kernel source by default, and only put in the linux/include directory. If you can't find the file in question, then install the kernel source and/or make use of the find and locate commands. If you can't find the kernel source package in your distribution then the kernel source is available at:

Kernel Source Home

The next best thing to reading the kernel C source itself, will be any of the other documentation files that are distributed with the kernel itself. There are now quite a few of these, and most of them can be found in the directory linux/Documentation and subdirectories from there. Sometimes there will be README.foo files that can be found in the related driver directory (e.g. linux/drivers/???/, where examples of ??? could be scsi, char, or net). The general trend is to move these files into the Documentation directory, so if a file mentioned in this document is no longer there, chances are it has been moved.

If you have figured out what boot-args you intend to use, and now want to know how to get that information to the kernel, then look at the documentation that comes with the software that you use to boot the kernel (e.g. LILO or loadlin). A brief overview is given below, but it is no substitute for the documentation that comes with the booting software.

1.3 New Versions of this Document

New versions of this document can be retrieved via anonymous FTP from most Linux FTP sites in the directory /pub/Linux/docs/HOWTO/. Updates will be made as new information and/or drivers becomes available. If this copy that you are presently reading is more than six months old, then you should probably check to see if a newer copy exists. I would recommend viewing this via a WWW browser or in the Postscript/dvi format. Both of these contain cross-references that are lost in a simple plain text version.

If you want to get the official copy, here is URL.

BootPrompt-HOWTO


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