89 lines
3.6 KiB
Groff
89 lines
3.6 KiB
Groff
.\" $NetBSD: mbr.8,v 1.5 2001/11/16 11:36:09 wiz Exp $
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.Dd April 13, 1999
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.Dt MBR 8 i386
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.Os
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.Sh NAME
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.Nm mbr ,
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.Nm bootselect
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.Nd
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Master Boot Record bootcode
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.Sh DESCRIPTION
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A PC boots from a disk by loading its first sector and executing the code
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in it. For a hard disk, this first sector usually contains a table of
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partitions present on the disk. The first sector of a disk containing such
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a table is called the Master Boot Record (MBR). The code present in
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the MBR will typically examine the partition table, find the partition
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that is marked active, and boot from it. Booting from a partition simply
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means loading the first sector in that partition, and executing the
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code in it, as is done for the MBR itself.
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.Pp
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.Nx
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supplies its two versions of the bootcode in an MBR:
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.Bl -tag -width Er
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.It Sy Normal boot code
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This version has the same functionality as that supplied by DOS/Windows and
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other operating systems: it picks the active partition and boots from it.
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Its advantage over other, older MBRs, is that it can detect and use
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extensions to the BIOS interface that will allow it to boot partitions
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that cross or start beyond the 8 Gigabyte boundary.
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.It Sy Bootselector
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The bootselecting MBR contains configurable code that will present
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the user with a simple menu, allowing a choice between partitions to
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boot from, and hard disks to boot from. The choices and default settings
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can be configured through
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.Xr fdisk 8 .
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.El
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.Pp
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The rest of this manual page will discuss the bootselecting version of
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the MBR. The configurable items of the bootselector are:
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.Bl -tag -width Er
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.It timeout
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The number of seconds that the bootcode will wait for the user to
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press a key, selecting a menu item. Must be in the range 0-3600.
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.It default
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The default partition or disk to boot from, should the timeout
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expire.
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.It active
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Activate/deactivate the bootselector. When deactivated, the
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bootcode will pick the first active partition, as usual.
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.El
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.Pp
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When active, the bootselector will let the user select the partition
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or drive to boot from via the keyboard. Keys F1 through F4 will
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initiate a startup from partitions 1 through 4 in the partition table,
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if the corresponding entries in the bootselector menu are active.
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.Pp
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Keys F5 through F10 will boot from harddisks 0 through 5 (BIOS numbers
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0x80 through 0x85). Booting from a drive is simply done by reading
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the MBR of that drive and executing it, so the bootcode present in
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the MBR of the chosen drive determines which partition (if any) will
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be booted in the end.
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.Pp
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The Enter key will cause the bootcode to find the active partition,
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and boot from it. If no key is pressed, the (configurable) default
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selection is picked.
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.Sh DIAGNOSTICS
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The following error numbers may be printed in case of an error condition,
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(after which the user must restart the system):
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.Bl -tag -width Er
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.It 1
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Invalid partition table. The MBR has a partition table without an
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active partition.
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.It 2
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Read error. There was an error reading the bootsector for the
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partition or drive selected.
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.It 3
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No operating system. The bootsector was loaded successfully, but
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it was not valid (i.e. the magic number check failed).
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.El
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.Sh SEE ALSO
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.Xr fdisk 8
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.Sh BUGS
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The bootselect code has constraints because of the limited amount of
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space available. The only way to be absolutely sure that a bootselector
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will always fit on the disk when a partition table is used, is to make
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it small enough to fit into the first sector (512 bytes, 446 not counting
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the partition table itself). The error messages are short. The code
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will give up if an error occurs, it should allow the user to, for example,
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specify another partition/disk instead.
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