Tài liệu Chapter-29-Starting and stopping the system doc

• On page 525 we’ll look at how to control the boot process.. • The second-level boot locates the third-level bootstrap, called loader, and loads it into memory.. By default, loader loca

Trang 1

Star ting and stopping the system

In this chapter:

• Star ting the system

• Things you can do

• Shutting down and

rebooting the system

• Star ting the system

• Things you can do

• Shutting down and

rebooting the system

• In the next section, we’ll look at the startup process in more detail

• On page 525 we’ll look at how to control the boot process

• If something goes wrong, and the system doesn’t come up, those messages thatscrolled off the screen are very important We’ll look at what they mean on page 525

• It’s not so obvious that you need to adhere to a procedure when shutting down thesystem We’ll look at the hows and whys on page 537

• There are a number of ways of starting the system for particular applications Onpage 538 we’ll look at how to run FreeBSD without a disk

starting.mm,v v4.19 (2003/04/02 04:32:28) 523

Trang 2

The Complete FreeBSD 524

Star ting the system

When you power up the system, or when you reboot, a number of actions occur beforethe system is up and running Starting the system is usually called ‘‘bootstrapping,’’ afterthe story of Baron von Munchhausen pulling himself up by his own bootstraps Thefollowing sequence describes the sequence on the PC architecture, but there are onlyrelatively minor differences on other platforms

• First, the BIOS1performs tests that check that the machine is working correctly and

determines the hardware configuration This Power On Self Test or POST has

nothing to do with FreeBSD

• Next, the BIOS bootstrap loads the Master Boot Record from the first sector of the

first disk on the system (C:in BIOS parlance) into memory and executes it Thisstep is the same for all operating systems on PCs

• It’s up to this bootstrap to decide which operating system to boot The bootstrap inthe MBR may or may not be part of the FreeBSD system FreeBSD can install twodifferent MBRs, as we saw on page 66 The standard MBR works withoutintervention, while the boot manager gives you the choice of booting from anypartition on the disk

• The FreeBSD bootstrap first loads the second-level bootstrap, BTX, from the next 15

sectors on disk and executes it

• The second-level boot locates the third-level bootstrap, called loader, and loads it into memory loader is an intelligent bootstrap component that allows preloading of multiple kernel components See the man page loader(8) for more information By default, loader locates the kernel, the file /boot/kernel/kernel on the root file system,

and loads it into memory You can interrupt the loader at this point, for example toload different or additional files

• The kernel performs its own tests to look for hardware it knows about It’s quiteverbose about this, and prints messages about both the hardware it finds and the

hardware it doesn’t find This operation is called probing Most kernels are built to

recognize a large selection of hardware, so it’s normal to have more ‘‘not found’’messages than ‘‘found’’ messages

• After the probe, the kernel starts two processes The first, process 0, is the swapper

and is responsible for emergency clearing of memory when the standard virtualmemory algorithms aren’t fast enough

• Process 1 is called init As the name suggests, it is responsible for starting up the

system and daemons When coming up in the default multi-user mode, it spawns a

shell to execute the shell script /etc/rc.

1 More accurately, the system firmware The firmware is called BIOS (Basic Input/Output System) on the i386 architecture, SRM on the Alpha architecture, and Open Firmware on a number of other architectures.

starting.mm,v v4.19 (2003/04/02 04:32:28)

Trang 3

• /etc/rc first reads in the description files /etc/defaults/rc.conf, which contains defaults

for a number of configuration variables, and /etc/rc.conf, which contains your

modifications to the defaults It then proceeds to perform the steps necessary to bring

up the system, first starting virtual disk drivers, mounting swap space and checkingthe file system integrity if necessary

• When /etc/rc exits, init reads the file /etc/ttys and starts processes as determined

there It spends the rest of its life looking after these processes

Things you can do before booting

You can do a number of things before you boot the system:

• The most obvious thing to do is to decide what you’re going to boot The boot loadergives you the chance to load different operating systems or different FreeBSD kernelsand modules We’ll look at that below

• In addition, you can set a number of options for the kernel loader, includingspecification of hardware and software characteristics We’ll look at that on page527

What are you going to boot?

If you have multiple operating systems on your system, you can use the boot managerdescribed on page 64, to choose which one to boot For example, if you have two disks,the first of which contains four partitions, the first stage of the boot looks something likethis:

key If you press F5, you get a menu showing the partitions on the second disk, with F5

to return to the first disk

If you choose to boot FreeBSD, the bootstrap loaders load, and you see something like:

BTX loader 1.00 BTX version is 1.01

BIOS drive A: is disk0

BIOS drive C: is disk1

BIOS drive D: is disk1

starting.mm,v v4.19 (2003/04/02 04:32:28)

Trang 4

What are you going to boot? 526

BIOS 639kB/130048kB available memory

These messages are printed by BTX If you’re loading from disk, the/character at theend of the previous line keeps changing through-,\, and|before going back to/again,

giving the impression that the character is rotating This display, called a twirling baton,

is your indication that the system hasn’t crashed and burned It’s normal for it to take afew seconds before the baton starts to twirl

Next, loader prints its prompt:

FreeBSD/i386 bootstrap loader, Revision 0.8

(grog@freebie.example.com, Thu Jun 13 13:06:03 CST 2002)

Loading /boot/defaults/loader.conf

Hit [Enter] to boot immediately, or any other key for command prompt.

Booting [kernel] in 6 seconds this counts down from 10 seconds

At this point, you would normally continue with the boot, either by pressing the Enter

key or just waiting the 10 seconds We’ll see what happens then on page 529

Sometimes you may want to change software or hardware characteristics In this case,you press the ‘‘any other key’’ (by tradition the space bar) and enter commands to theloader

Loader commands

There are two ways to communicate with the loader:

• A number of files in the directory /boot on the root file system tell the loader what to

do Most are not intended to be changed, but you can create a file called

/boot/loader.conf, into which you can enter commands to override the commands in /boot/defaults/loader.conf We’ll look at this below.

• In addition, the file /boot/device.hints takes the place of many configuration file

entries and allows you to set hardware characteristics such as information about IRQ,DMA, I/O address and other settings for the hardware You can change these valuesduring booting

The CD-ROM installation installs /boot/device.hints, but a kernel install does not.

You’ll find it in the conf directory for your architecture For example,

/usr/src/sys/i386/conf includes the configuration file GENERIC and the

correspond-ing hints file GENERIC.hints Install it like this:

Trang 5

device sio0 at isa? port IO_COM1 flags 0x10 irq 4

device sio1 at isa? port IO_COM2 irq 3

device sio2 at isa? disable port IO_COM3 irq 5

device sio3 at isa? disable port IO_COM4 irq 9

The corresponding line in the Release 5 configuration file is:

More importantly, though, this means that you don’t need to recompile the kernel ifyou change the hardware addresses

• You can enter commands directly to the command prompt

When you hit the space bar, you get the following prompt:

Type ’?’ for a list of commands, ’help’ for more detailed help.

ok ?

Available commands:

reboot reboot the system

heap show heap usage

bcachestat get disk block cache stats

boot boot a file or loaded kernel

autoboot boot automatically after a delay

help detailed help

show show variable(s)

set set a variable

unset unset a variable

Định dạng
Số trang	23
Dung lượng	228,55 KB

Tiêu đề	Starting and stopping the system
Thể loại	Chapter
Năm xuất bản	2003