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Explain how the kernel determines the root filesystem from information 

passed by the bootloader.
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mlelstv 2015-04-25 14:05:43 +00:00
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52
share/man/man8/man8.x86/boot.8
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@ -1,4 +1,4 @@
.\" $NetBSD: boot.8,v 1.4 2014/08/16 12:30:12 apb Exp $
.\" $NetBSD: boot.8,v 1.5 2015/04/25 14:05:43 mlelstv Exp $
.\"
.\" Copyright (c) 1991, 1993
.\" The Regents of the University of California. All rights reserved.
@ -624,6 +624,55 @@ In an emergency, the bootstrap methods described in the
installation notes for the x86 architectures
can be used to boot from floppy or other media,
or over the network.
.Ss Locating the root filesystem
The kernel uses information from the bootloader to locate the
filesystem to mount as root. There are three methods:
.Bl -tag -width 04n -offset 04n
.It Ic BTINFO_ROOTDEVICE Va from
.Xr boot.cfg 5
or multiboot.
The bootloader passes the root device name as driver,unit and
partition (like sd0a). This will be automatically substituted
by a
.Xr dk 4
wedge if one is discovered.
.Pp
The bootloader passes a wedge name as "wedge:" followed by the name.
The kernel will search for a
.Xr dk 4
device with that name.
.It Ic BTINFO_BOOTWEDGE Va determined by bootblock
The bootloader passes start offset and length of a hard disk partition
and a offset, size and hash of a "boot area". Then kernel searches
all disks and wedges for a block sequence at that offset with a
matching hash. If one is found, the kernel will look for a wedge
on that device at the same offset.
.Pp
An additional partition number is provided if the bootloader also
passed a BTINFO_BOOTDISK record. This (or partition 'a') will be used
by the kernel as a fallback if there is no matching wedge.
.It Ic BTINFO_BOOTDISK Va determined by bootblock
This uses the device number passed by the BIOS that
distinguishes between floppy, hard drive and CD-ROM boot.
.Bl -tag -width xxx
.It Ic Floppy
The kernel searches for the
.Xr fd 4
device with the correct unit, the partition number is used
to select a specific disk format. See
.Xr fd 4 for details.
.It Ic Hard drive
The bootloader passed a partition number and disklabel
data (offset, type, checksum, packname). The kernel searches
all disks for a matching disklabel. If one is found, the
kernel will use that device and partition number.
.It Ic CDROM
The BIOS does not distinguish between multiple CD devices.
The kernel searches for the first
.Xr cd 4
device. So you can only boot from unit 0.
.El
.El
.Sh FILES
.Bl -tag -width /usr/mdec/bootxx_fstype -compact
.It Pa /boot
@ -647,6 +696,7 @@ partition by
.Xr ddb 4 ,
.Xr pciback 4 ,
.Xr userconf 4 ,
.Xr fd 4 ,
.Xr boot.cfg 5 ,
.Xr boot_console 8 ,
.Xr dosboot 8 ,