Linux内核启动参数详解

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"The Linux BootPrompt-HowTo 是一份由 Paul Gortmaker 编写的指南，专注于介绍在 Linux 启动时可以传递给内核的各种启动参数。这份文档包含了如何组织启动时间参数的讨论，以及一些流行的 Linux 引导软件的概述。" 本文档主要分为以下几个部分： 1. **引言** - **1.1 目标读者和适用性**：这部分可能阐述了这篇文档适用于哪些用户群体，例如系统管理员、开发者或对 Linux 启动过程感兴趣的用户。 - **1.2 相关文档**：可能提到了与 BootPrompt-HowTo 文档相关的其他技术资料，如 Linux 内核文档、引导加载器手册等。 - **1.3 新版本信息**：作者可能在这部分提及文档的更新历史和获取最新版本的方式。 2. **启动提示符参数概述** - **2.1 LILO (Linux Loader)**：LILO 是一个早期的 Linux 引导装载程序，用于加载操作系统内核。这里可能会讲解如何通过 LILO 设置启动参数。 - **2.2 LoadLin**：可能是一个辅助工具，用于在没有 LILO 或其他引导装载程序的情况下引导 Linux。 - **2.3 The ``rdev'' utility**：这可能是一个用于设置内核设备编号的工具，特别是在多引导系统中。 - **2.4 内核如何排序启动参数**：这部分解释了内核如何处理和解析在启动时传入的参数。 - **2.5 设置环境变量**：说明如何在启动时设置环境变量，这些变量会影响内核的行为。 - **2.6 将参数传递给 `init` 程序**：`init` 是 Linux 的第一个进程，这部分可能介绍了如何通过启动参数传递信息给 `init`。 3. **通用非设备特定的启动参数** - **3.1 根文件系统选项**：这部分列出了可以用于指定根文件系统的启动参数。 - **3.2 与 RAMDisk 管理相关的选项**：说明如何配置和使用 RAMDisk（内存中的虚拟磁盘）。 - **3.3 与内存处理相关的启动参数**：可能包括内存限制、交换空间设置等。 - **3.4 其他杂项内核启动参数**：涵盖了不归类于以上类别的各种内核启动选项。 4. **控制 PCI 总线行为的启动参数 (`pci=`)** - **4.1 The `pci=assign-busses' Argument**：此参数可能用于让内核在启动时显式分配 PCI 总线地址。 - **4.2 The `pci="..."' 参数**：这部分可能介绍了更多关于如何配置 PCI 设备和总线的高级选项。文档的剩余部分可能继续深入讨论了其他特定设备的启动参数，或者包含了一些实用示例和故障排除指南。这份详尽的 HowTo 文档对于理解和优化 Linux 系统的启动过程非常有价值，无论是对于新手还是经验丰富的系统管理员。

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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 use

DOS and want to launch into 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 set a few

proprietary 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.EXE

avoids the reset of the card that happens if one rebooted instead. Thus the card

is left in a SB compatible mode and hence is useable under Linux.

There are also other programs that can be used to boot Linux. For a complete list,

please look at the programs available on your local Linux ftp mirror, under

system/Linux-boot/.

2.3 The ``rdev'' utility

There are a few of the kernel boot parameters that have their default values

stored in various bytes in the kernel image itself. There is a utility called rdev

that is installed on most systems that knows where these values are, and how to

change them. It can also change things that have no kernel boot argument

equivalent, such as the default video mode used.

The rdev utility is usually also aliased to swapdev, ramsize, vidmode and

rootflags. These are the five things that rdev can change, those being the root

device, the swap device, the RAM disk parameters, the default video mode, and the

readonly/readwrite setting of root device.

More information on rdev can be found by typing rdev -h or by reading the supplied

man page (man rdev).

2.4 How the Kernel Sorts the Arguments

Most of the boot args take the form of:

name[=value_1][,value_2]...[,value_11]

where `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. Multiple boot args are just a

space separated list of the above format. Note the limit of 11 is real, as the

present code only handles 11 comma separated parameters per keyword. (However, you

can re-use the same keyword with up to an additional 11 parameters in unusually

complicated situations, assuming the setup function supports it.) Also note that

the kernel splits the list into a maximum of ten integer arguments, and a

following string, so you can't really supply 11 integers unless you convert the

11th arg from a string to an int in the driver itself.

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=', `ro', `rw', or `debug'. The

meaning of these special arguments is described further on in the document.

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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,bar 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 integer arguments 3, 4, 5 and 6

as given on the kernel command line, and also hand it the string argument bar.

2.5 Setting Environment Variables.

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. An

example would be to use TERM=vt100 or BOOT_IMAGE=vmlinuz.bak as a boot argument. These

environment variables are typically tested for in the initialization scripts to

enable or disable a wide range of things.

2.6 Passing Arguments to the `init' program

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.

3. General Non-Device Specific Boot Args

These are the boot arguments that are not related to any specific device or

peripheral. They are instead related to certain internal kernel parameters, such

as memory handling, ramdisk handling, root file system handling and others.

3.1 Root Filesystem options

The following options all pertain to how the kernel selects and handles the root

filesystem.

The `root=' Argument

This argument tells the kernel what device is to be used as the root filesystem

while booting. The default of this setting is the value of the root device of the

system that the kernel was built on. For example, if the kernel in question was

built on a system that used `/dev/hda1' as the root partition, then the default

root device would be `/dev/hda1'. To override this default value, and select the

second floppy drive as the root device, one would use `root=/dev/fd1'.

Valid root devices are any of the following devices:

(1) /dev/hdaN to /dev/hddN, which is partition N on ST-506 compatible disk `a to

d'.

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