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发信人: Lg (创造人生的传奇), 信区: Linux
标 题: The Boot Procedure(转寄)
发信站: BBS 荔园晨风站 (Fri Jan 15 23:36:47 1999), 站内信件
【 以下文字转载自 Lg 的信箱 】
【 原文由 liugang.bbs@bbs.net.tsinghua.edu.cn 所发表 】
发信人: reden (鱼 ～ 梦娜丽莎的微笑 流星的故事), 信区: Linux
标 题: The Boot Procedure(转寄)
发信站: BBS 水木清华站 (Wed Nov 25 01:13:49 1998)
Last modified: Thursday, 21 August 1997 15:09 EST
The Boot Procedure
Bootstrapping is the process of starting up a computer from a halted or
powered-down condition. When the computer is
switched on, it activates the memory-resident code which resides on the CPU
board. The normal facilities of the operating
system are not available at this stage and the computer must 'pull itself up by
its own boot-straps' so to speak. This
procedure therefore is often referred to as bootstrapping, also known as cold
boot. Although the bootstrap procedure is
very hardware dependent, it typically consists of the following steps:
The memory-resident code
Runs self-test.
Probes bus for the boot device
Reads the boot program from the boot device.
Boot program reads in the kernel and passes control to it.
Kernel identifies and configures the devices.
Initializes the system and starts the system processes.
Brings up the system in single-user mode (if necessary).
Runs the appropriate startup scripts.
Brings up the system for multi-user operation.
Kernel
Most Unix systems implement a two-stage loading process. During the first stage,
a small boot program is read into
memory from a default or specified device. It is this program that reads in the
kernel and relinquishes the control to it.
The path to the kernel is vendor-dependent. For example, it is /vmunix on SunOS
4.x, Digital Unix and Ultrix, /kernel/unix
on SunOS 5.x, /hp-ux on HP-UX, and /unix on IRIX and AIX systems.
One of the very first, probably the most difficult, tasks a system
administrator must perform, is configuring the kernel.
You'll read the details later in the sections dealing with the 'Operating
System Installation'. Once the kernel is loaded, it
remains in the memory during the running of the system and is usually run in a
fixed amount of memory. When the kernel
starts, it normally displays its size and the amount of physical memory
remaining after the kernel is loaded. The kernel
probes the bus to locate the devices specified during the configuration, and
initializes the located devices (ignoring those
that it can't contact). Any device not detected and initialized during the boot
will not be accessible to system until it is
properly connected and the system is rebooted.
System Processes
The kernel identifies the root, swap, and dump devices and then starts programs
to schedule processes, manage
physical memory and virtual memory, and the init process. BSD systems starts
three initialization processes; swapper, init
and pagedaemon. On the SVR4 systems the initialization processes include sched,
init, and various memory handlers
(except on Solaris).
sched
The real-time scheduler, sched, runs as process 0 on SVR5 systems. It can be
used to set priority for real-time
processes so that they can be given fast access to the kernel.
swapper
The swapper daemon runs as process 0 on BSD systems. It manages the physical
memory by moving process from
physical memory to swap space when more physical memory is needed.
Page Daemon
Various memory handlers run as process 2. When a page of virtual memory is
accessed, the page table within the kernel
is consulted and if necessary, the pagedaemon (SunOS 4.x) or pageout (SunOS 5.x)
is used to move pages in and out of
physical memory and to update page tables. Similar memory handlers exist on
other SVR5 systems.
init
The last step in bootstrapping the kernel starts the /etc/init process. The
init process runs as process 1 and always
remains in the background when the system is running. If the system is brought
up in a single user mode, init merely
creates a shell on the system console (/dev/console) and waits for it to
terminate before running other startup scripts.
Single User Mode
Single user shell is always Bourne shell (sh) and it runs as 'root'. It enables
the system manager to perform various
administrative functions, such as setting the date, checking the consistency of
the file system, reconfiguring the list of
on-line terminals, and so on. At this stage only the root partition is usually
mounted. Other file systems will have to be
mounted manually to use programs that do not reside on the root volume. The
file system consistency check may be
performed by the command fsck, usually found in the /etc directory.
Startup Scripts
The startup scripts are merely shell scripts, so init spawns a copy of sh to
interpret them. The startup scripts are defined
and organized differently on different systems. On BSD systems the startup
scripts may be found in the /etc directory and
their names begin with rc, e.g., /etc/rc.boot, /etc/rc.single, /etc/rc.local
and so on. SVR5 systems define various run levels
in which a specific set of processes are allowed to run. This set of processes
is defined in the /etc/inittab file. Each line in
the inittab file describes an action to take. The syntax of inittab entries is:
id:run-level:action:process
id uniquely identifies the entry. It may be a one or characters string.
run-level defines the run level in which the entry can be processed. If
this field is empty, all run levels are
assumed.
action identifies what actions to take for this entry. These actions may
include:
sysinit - perform system initialization,
wait - wait for the process to complete,
once - run the process only once,
respawn - restart the process whenever it dies.
process specifies the shell command to be run, if the entry's run level
matches the run level, and/or the action
field indicates such action.
In general, the following tasks are performed in the startup scripts.
Set the hostname.
Set the time zone.
Run the file system consistency check.
Mount the system's disk partitions.
Start the daemons and network services.
Configure the network interface.
Turn on the accounting and quotas.
Terms used: boot, bootstrapping kernel, init,swap, virtual memory, run level.
Last modified: Thursday, 21 August 1997 15:09
EST
<URL:http://www.uwsg.indiana.edu/usail/installation/boot-process.html>
Copyright 1996, The Trustees of Indiana
University
Comments and suggestions to:
usail@indiana.edu
--
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但终是能回到中原的。江南有杨柳、桃花，有燕子、金鱼……
汉人中有的是英俊勇武的少年，倜傥潇洒的少年……但这个美
丽的姑娘就像古高昌国人那样固执:
「那都是很好很好的，可是我偏不喜欢。」
※ 来源:·BBS 水木清华站 bbs.net.tsinghua.edu.cn·[FROM: 159.226.21.172]
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