add directory study
This commit is contained in:
gohigh
401
study/boot/Bootdisk-HOWTO/Bootdisk-HOWTO-v450/x88.html
Normal file
401
study/boot/Bootdisk-HOWTO/Bootdisk-HOWTO-v450/x88.html
Normal file
@@ -0,0 +1,401 @@
|
||||
<HTML
|
||||
><HEAD
|
||||
><TITLE
|
||||
>Bootdisks and the boot process</TITLE
|
||||
><META
|
||||
NAME="GENERATOR"
|
||||
CONTENT="Modular DocBook HTML Stylesheet Version 1.63
|
||||
"><LINK
|
||||
REL="HOME"
|
||||
TITLE="The Linux Bootdisk HOWTO"
|
||||
HREF="index.html"><LINK
|
||||
REL="PREVIOUS"
|
||||
TITLE="Introduction"
|
||||
HREF="x68.html"><LINK
|
||||
REL="NEXT"
|
||||
TITLE="Building a root filesystem"
|
||||
HREF="buildroot.html"></HEAD
|
||||
><BODY
|
||||
CLASS="SECT1"
|
||||
BGCOLOR="#FFFFFF"
|
||||
TEXT="#000000"
|
||||
LINK="#0000FF"
|
||||
VLINK="#840084"
|
||||
ALINK="#0000FF"
|
||||
><DIV
|
||||
CLASS="NAVHEADER"
|
||||
><TABLE
|
||||
WIDTH="100%"
|
||||
BORDER="0"
|
||||
CELLPADDING="0"
|
||||
CELLSPACING="0"
|
||||
><TR
|
||||
><TH
|
||||
COLSPAN="3"
|
||||
ALIGN="center"
|
||||
>The Linux Bootdisk HOWTO</TH
|
||||
></TR
|
||||
><TR
|
||||
><TD
|
||||
WIDTH="10%"
|
||||
ALIGN="left"
|
||||
VALIGN="bottom"
|
||||
><A
|
||||
HREF="x68.html"
|
||||
>Prev</A
|
||||
></TD
|
||||
><TD
|
||||
WIDTH="80%"
|
||||
ALIGN="center"
|
||||
VALIGN="bottom"
|
||||
></TD
|
||||
><TD
|
||||
WIDTH="10%"
|
||||
ALIGN="right"
|
||||
VALIGN="bottom"
|
||||
><A
|
||||
HREF="buildroot.html"
|
||||
>Next</A
|
||||
></TD
|
||||
></TR
|
||||
></TABLE
|
||||
><HR
|
||||
ALIGN="LEFT"
|
||||
WIDTH="100%"></DIV
|
||||
><DIV
|
||||
CLASS="SECT1"
|
||||
><H1
|
||||
CLASS="SECT1"
|
||||
><A
|
||||
NAME="AEN88"
|
||||
>3. Bootdisks and the boot process</A
|
||||
></H1
|
||||
><P
|
||||
>A bootdisk is basically a miniature, self-contained Linux system on a
|
||||
diskette. It must perform many of the same functions that a complete
|
||||
full-size Linux system performs. Before trying to build one you should
|
||||
understand the basic Linux boot process. Here we present the basics, which
|
||||
are sufficient for understanding the rest of this document. Many details and
|
||||
alternative options have been omitted.</P
|
||||
><DIV
|
||||
CLASS="SECT2"
|
||||
><H2
|
||||
CLASS="SECT2"
|
||||
><A
|
||||
NAME="AEN93"
|
||||
>3.1. The boot process</A
|
||||
></H2
|
||||
><P
|
||||
>
|
||||
|
||||
All PC systems start the boot process by executing code in ROM
|
||||
(specifically, the <EM
|
||||
>BIOS</EM
|
||||
>) to load the sector from
|
||||
sector 0, cylinder 0 of the boot drive. The boot drive is usually the
|
||||
first floppy drive (designated <TT
|
||||
CLASS="FILENAME"
|
||||
>A:</TT
|
||||
> in DOS and
|
||||
<TT
|
||||
CLASS="FILENAME"
|
||||
>/dev/fd0</TT
|
||||
> in Linux). The BIOS then tries to execute
|
||||
this sector. On most bootable disks, sector 0, cylinder 0 contains either:
|
||||
|
||||
<P
|
||||
></P
|
||||
><UL
|
||||
><LI
|
||||
><P
|
||||
>code from a boot loader such as LILO, which locates the kernel,
|
||||
loads it and executes it to start the boot proper; or </P
|
||||
></LI
|
||||
><LI
|
||||
><P
|
||||
>the start of an operating system kernel, such as Linux. </P
|
||||
></LI
|
||||
></UL
|
||||
> </P
|
||||
><P
|
||||
>If a Linux kernel has been raw-copied to a diskette, the first sector of the
|
||||
disk will be the first sector of the Linux kernel itself. This first sector
|
||||
will continue the boot process by loading the rest of the kernel from the boot
|
||||
device.</P
|
||||
><P
|
||||
>When the kernel is completely loaded, it initializes device drivers and its
|
||||
internal data structures. Once it is completely initialized, it consults a
|
||||
special location in its image called the <EM
|
||||
>ramdisk word</EM
|
||||
>.
|
||||
This word tells it how and where to find its <EM
|
||||
>root
|
||||
filesystem</EM
|
||||
>. A root filesystem is simply a filesystem that will be
|
||||
mounted as ``<TT
|
||||
CLASS="FILENAME"
|
||||
>/</TT
|
||||
>''. The kernel has to be told where to
|
||||
look for the root filesystem; if it cannot find a loadable image there, it
|
||||
halts.</P
|
||||
><P
|
||||
>In some boot situations — often when booting from a diskette
|
||||
— the root filesystem is loaded into a <EM
|
||||
>ramdisk</EM
|
||||
>,
|
||||
which is RAM accessed by the system as if it were a disk. RAM is several
|
||||
orders of magnitude faster than a floppy disk, so system operation is fast
|
||||
from a ramdisk. Also, the kernel can load a <EM
|
||||
>compressed
|
||||
filesystem</EM
|
||||
> from the floppy and uncompress it onto the ramdisk,
|
||||
allowing many more files to be squeezed onto the diskette. </P
|
||||
><P
|
||||
>Once the root filesystem is loaded and mounted, you see a message like:
|
||||
<TABLE
|
||||
BORDER="1"
|
||||
BGCOLOR="#E0E0E0"
|
||||
WIDTH="100%"
|
||||
><TR
|
||||
><TD
|
||||
><FONT
|
||||
COLOR="#000000"
|
||||
><PRE
|
||||
CLASS="SCREEN"
|
||||
> VFS: Mounted root (ext2 filesystem) readonly.</PRE
|
||||
></FONT
|
||||
></TD
|
||||
></TR
|
||||
></TABLE
|
||||
></P
|
||||
><P
|
||||
>Once the system has loaded a root filesystem successfully, it tries to
|
||||
execute the <TT
|
||||
CLASS="FILENAME"
|
||||
>init</TT
|
||||
> program (in <TT
|
||||
CLASS="FILENAME"
|
||||
>/bin</TT
|
||||
> or
|
||||
<TT
|
||||
CLASS="FILENAME"
|
||||
>/sbin</TT
|
||||
>). <TT
|
||||
CLASS="FILENAME"
|
||||
>init</TT
|
||||
> reads its
|
||||
configuration file <TT
|
||||
CLASS="FILENAME"
|
||||
>/etc/inittab</TT
|
||||
>, looks for a line
|
||||
designated <TT
|
||||
CLASS="LITERAL"
|
||||
>sysinit</TT
|
||||
>, and executes the named script. The
|
||||
<TT
|
||||
CLASS="LITERAL"
|
||||
>sysinit</TT
|
||||
> script is usually something like
|
||||
<TT
|
||||
CLASS="FILENAME"
|
||||
>/etc/rc</TT
|
||||
> or <TT
|
||||
CLASS="FILENAME"
|
||||
>/etc/init.d/boot</TT
|
||||
>. This
|
||||
script is a set of shell commands that set up basic system services, such as
|
||||
running <B
|
||||
CLASS="COMMAND"
|
||||
>fsck</B
|
||||
> on hard disks, loading necessary kernel
|
||||
modules, initializing swapping, initializing the network, and mounting disks
|
||||
mentioned in <TT
|
||||
CLASS="FILENAME"
|
||||
>/etc/fstab</TT
|
||||
>.</P
|
||||
><P
|
||||
>This script often invokes various other scripts to do modular
|
||||
initialization. For example, in the common SysVinit structure, the directory
|
||||
<TT
|
||||
CLASS="FILENAME"
|
||||
>/etc/rc.d/</TT
|
||||
> contains a complex structure of subdirectories
|
||||
whose files specify how to enable and shut down most system services. However,
|
||||
on a bootdisk the sysinit script is often very simple.</P
|
||||
><P
|
||||
>When the sysinit script finishes control returns to <B
|
||||
CLASS="COMMAND"
|
||||
>init</B
|
||||
>,
|
||||
which then enters the <EM
|
||||
>default runlevel</EM
|
||||
>, specified in
|
||||
<TT
|
||||
CLASS="FILENAME"
|
||||
>inittab</TT
|
||||
> with the <TT
|
||||
CLASS="LITERAL"
|
||||
>initdefault</TT
|
||||
> keyword.
|
||||
The runlevel line usually specifies a program like <B
|
||||
CLASS="COMMAND"
|
||||
>getty</B
|
||||
>,
|
||||
which is responsible for handling commununications through the console and
|
||||
ttys. It is the <B
|
||||
CLASS="COMMAND"
|
||||
>getty</B
|
||||
> program which prints the familiar
|
||||
``<TT
|
||||
CLASS="PROMPT"
|
||||
>login:</TT
|
||||
>'' prompt. The <B
|
||||
CLASS="COMMAND"
|
||||
>getty</B
|
||||
> program in
|
||||
turn invokes the <B
|
||||
CLASS="COMMAND"
|
||||
>login</B
|
||||
> program to handle login validation
|
||||
and to set up user sessions.</P
|
||||
></DIV
|
||||
><DIV
|
||||
CLASS="SECT2"
|
||||
><H2
|
||||
CLASS="SECT2"
|
||||
><A
|
||||
NAME="AEN159"
|
||||
>3.2. Disk types</A
|
||||
></H2
|
||||
><P
|
||||
>Having reviewed the basic boot process, we can now define various
|
||||
kinds of disks involved. We classify disks into four types. The
|
||||
discussion here and throughout this document uses the term ``disk'' to
|
||||
refer to floppy diskettes unless otherwise specified, though most of
|
||||
the discussion could apply equally well to hard disks.</P
|
||||
><P
|
||||
><P
|
||||
></P
|
||||
><DIV
|
||||
CLASS="VARIABLELIST"
|
||||
><DL
|
||||
><DT
|
||||
>boot</DT
|
||||
><DD
|
||||
><P
|
||||
>A disk containing a kernel which can be booted. The disk
|
||||
can be used to boot the kernel, which then may load a root file system on
|
||||
another disk. The kernel on a bootdisk usually must be told where to find
|
||||
its root filesystem.</P
|
||||
><P
|
||||
>Often a bootdisk loads a root filesystem from another diskette, but it is
|
||||
possible for a bootdisk to be set up to load a hard disk's root filesystem
|
||||
instead. This is commonly done when testing a new kernel (in fact,
|
||||
``<B
|
||||
CLASS="COMMAND"
|
||||
>make zdisk</B
|
||||
>'' will create such a bootdisk automatically
|
||||
from the kernel source code).</P
|
||||
></DD
|
||||
><DT
|
||||
>root</DT
|
||||
><DD
|
||||
><P
|
||||
>A disk with a filesystem containing files
|
||||
required to run a Linux system. Such a disk does not necessarily
|
||||
contain either a kernel or a boot loader. </P
|
||||
><P
|
||||
>A root disk can be used to run the system independently of any other
|
||||
disks, once the kernel has been booted. Usually the root disk is
|
||||
automatically copied to a ramdisk. This makes root disk accesses much
|
||||
faster, and frees up the disk drive for a utility disk.</P
|
||||
></DD
|
||||
><DT
|
||||
>boot/root</DT
|
||||
><DD
|
||||
><P
|
||||
>A disk which contains both the kernel and a root filesystem. In other
|
||||
words, it contains everything necessary to boot and run a Linux system
|
||||
without a hard disk. The advantage of this type of disk is that is it
|
||||
compact — everything required is on a single disk. However, the gradually
|
||||
increasing size of everything means that it is increasingly difficult to
|
||||
fit everything on a single diskette, even with compression.</P
|
||||
></DD
|
||||
><DT
|
||||
>utility</DT
|
||||
><DD
|
||||
><P
|
||||
>A disk which contains a filesystem, but is not
|
||||
intended to be mounted as a root file system. It is an additional data
|
||||
disk. You would use this type of disk to carry additional utilities where
|
||||
you have too much to fit on your root disk. </P
|
||||
></DD
|
||||
></DL
|
||||
></DIV
|
||||
></P
|
||||
><P
|
||||
>In general, when we talk about ``building a bootdisk'' we mean
|
||||
creating both the boot (kernel) and root (files) portions. They may
|
||||
be either together (a single boot/root disk) or separate (boot + root
|
||||
disks). The most flexible approach for rescue diskettes is probably
|
||||
to use separate boot and root diskettes, and one or more utility
|
||||
diskettes to handle the overflow.</P
|
||||
></DIV
|
||||
></DIV
|
||||
><DIV
|
||||
CLASS="NAVFOOTER"
|
||||
><HR
|
||||
ALIGN="LEFT"
|
||||
WIDTH="100%"><TABLE
|
||||
WIDTH="100%"
|
||||
BORDER="0"
|
||||
CELLPADDING="0"
|
||||
CELLSPACING="0"
|
||||
><TR
|
||||
><TD
|
||||
WIDTH="33%"
|
||||
ALIGN="left"
|
||||
VALIGN="top"
|
||||
><A
|
||||
HREF="x68.html"
|
||||
>Prev</A
|
||||
></TD
|
||||
><TD
|
||||
WIDTH="34%"
|
||||
ALIGN="center"
|
||||
VALIGN="top"
|
||||
><A
|
||||
HREF="index.html"
|
||||
>Home</A
|
||||
></TD
|
||||
><TD
|
||||
WIDTH="33%"
|
||||
ALIGN="right"
|
||||
VALIGN="top"
|
||||
><A
|
||||
HREF="buildroot.html"
|
||||
>Next</A
|
||||
></TD
|
||||
></TR
|
||||
><TR
|
||||
><TD
|
||||
WIDTH="33%"
|
||||
ALIGN="left"
|
||||
VALIGN="top"
|
||||
>Introduction</TD
|
||||
><TD
|
||||
WIDTH="34%"
|
||||
ALIGN="center"
|
||||
VALIGN="top"
|
||||
> </TD
|
||||
><TD
|
||||
WIDTH="33%"
|
||||
ALIGN="right"
|
||||
VALIGN="top"
|
||||
>Building a root filesystem</TD
|
||||
></TR
|
||||
></TABLE
|
||||
></DIV
|
||||
></BODY
|
||||
></HTML
|
||||
>
|
||||
Reference in New Issue
Block a user