NetBSD/sbin/fdisk/mbr/mbr.8

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.\" $NetBSD: mbr.8,v 1.8 2002/07/09 22:51:13 wiz Exp $
.Dd April 13, 1999
.Dt MBR 8
.Os
.Sh NAME
.Nm mbr ,
.Nm bootselect
.Nd Master Boot Record bootcode
.Sh DESCRIPTION
An IBM PC boots from a disk by loading its first sector and executing
the code in it.
For a hard disk, this first sector usually contains a table of
partitions present on the disk.
The first sector of a disk containing such
a table is called the Master Boot Record (MBR).
.Pp
The code present in the MBR will typically examine the partition
table, find the partition that is marked active, and boot from it.
Booting from a partition simply means loading the first sector in
that partition, and executing the code in it, as is done for the
MBR itself.
.Pp
.Nx
supplies its two versions of the bootcode in an MBR:
.Bl -tag -width Er
.It Sy Normal boot code
This version has the same functionality as that supplied by DOS/Windows and
other operating systems: it picks the active partition and boots from it.
Its advantage over other, older MBRs, is that it can detect and use
extensions to the BIOS interface that will allow it to boot partitions
that cross or start beyond the 8 Gigabyte boundary.
.It Sy Bootselector
The bootselecting MBR contains configurable code that will present
the user with a simple menu, allowing a choice between partitions to
boot from, and hard disks to boot from.
The choices and default settings can be configured through
.Xr fdisk 8 .
.El
.Pp
The rest of this manual page will discuss the bootselecting version of
the MBR.
The configurable items of the bootselector are:
.Bl -tag -width Er
.It timeout
The number of seconds that the bootcode will wait for the user to
press a key, selecting a menu item.
Must be in the range 0-3600.
.It default
The default partition or disk to boot from, should the timeout
expire.
.It active
Activate/deactivate the bootselector.
When deactivated, the bootcode will pick the first active partition,
as usual.
.El
.Pp
When active, the bootselector will let the user select the partition
or drive to boot from via the keyboard.
.Pp
Function keys
.Sy F1
through
.Sy F4
will initiate a startup from partitions 1 through 4 in the partition
table, if the corresponding entries in the bootselector menu are active.
.Pp
Function keys
.Sy F5
through
.Sy F10
will boot from harddisks 0 through 5 (BIOS numbers 0x80 through 0x85).
Booting from a drive is simply done by reading the MBR of that
drive and executing it, so the bootcode present in the MBR of the
chosen drive determines which partition (if any) will be booted in
the end.
.Pp
The
.Sy Enter
key will cause the bootcode to find the active partition, and boot from it.
If no key is pressed, the (configurable) default selection is picked.
.Sh DIAGNOSTICS
The following error numbers may be printed in case of an error condition,
(after which the user must restart the system):
.Bl -tag -width Er
.It 1
Invalid partition table.
The MBR has a partition table without an active partition.
.It 2
Read error.
There was an error reading the bootsector for the partition or
drive selected.
.It 3
No operating system.
The bootsector was loaded successfully, but it was not valid (i.e.
the magic number check failed).
.El
.Sh SEE ALSO
.Xr fdisk 8 ,
.Xr mbrlabel 8
.Sh BUGS
The bootselect code has constraints because of the limited amount of
space available.
The only way to be absolutely sure that a bootselector will always
fit on the disk when a partition table is used, is to make it small
enough to fit into the first sector (512 bytes, 446 not counting
the partition table itself).
.Pp
The error messages are short.
.Pp
The code will give up if an error occurs, it should allow the user
to, for example, specify another partition/disk instead.