mbr(8) - NetBSD Manual Pages

MBR(8)               NetBSD/i386  System Manager's Manual               MBR(8)


NAME

     mbr, bootselect - Master Boot Record bootcode


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).

     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.

     NetBSD supplies its two versions of the bootcode in an MBR:

     Normal boot code
                   This version has the same functionality as that supplied by
                   DOS/Windows and other operating systems: it picks the ac-
                   tive partition and boots from it.  Its advantage over oth-
                   er, 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.

     Bootselector  The bootselecting MBR contains configurable code that will
                   present the user with a simple menu, allowing a choice be-
                   tween partitions to boot from, and hard disks to boot from.
                   The choices and default settings can be configured through
                   fdisk(8).

     The rest of this manual page will discuss the bootselecting version of
     the MBR.  The configurable items of the bootselector are:

     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.

     default       The default partition or disk to boot from, should the
                   timeout expire.

     active        Activate/deactivate the bootselector.  When deactivated,
                   the bootcode will pick the first active partition, as usu-
                   al.

     When active, the bootselector will let the user select the partition or
     drive to boot from via the keyboard.

     Function keys F1 through F4 will initiate a startup from partitions 1
     through 4 in the partition table, if the corresponding entries in the
     bootselector menu are active.

     Function keys F5 through F10 will boot from harddisks 0 through 5 (BIOS
     numbers 0x80 through 0x85).  Booting from a drive is simply done by read-
     ing 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.

     The 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.


DIAGNOSTICS

     The following error numbers may be printed in case of an error condition,
     (after which the user must restart the system):

     1             Invalid partition table.  The MBR has a partition table
                   without an active partition.

     2             Read error.  There was an error reading the bootsector for
                   the partition or drive selected.

     3             No operating system.  The bootsector was loaded successful-
                   ly, but it was not valid (i.e.  the magic number check
                   failed).


SEE ALSO

     fdisk(8), mbrlabel(8)


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).

     The error messages are short.

     The code will give up if an error occurs, it should allow the user to,
     for example, specify another partition/disk instead.

NetBSD 1.6                      April 13, 1999                               2