mbr(8) - NetBSD Manual Pages

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MBR(8)                NetBSD/x86 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 several versions of the MBR bootcode:

     Normal boot code /usr/mdec/mbr
         This version has the same functionality as that supplied by DOS/Win-
         dows 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.

     Bootselector /usr/mdec/mbr_bootsel
         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 set-
         tings can be configured through fdisk(8).

     Extended Bootselector /usr/mdec/mbr_ext
         The extended bootselecting MBR additionally allows NetBSD to be
         loaded from an Extended partition.  It only supports systems whose
         BIOS supports the extensions to boot partitions beyond the 8 Gigabyte
         boundary.

     Serial Bootselector /usr/mdec/mbr_com0
         This has the same features as mbr_ext but will read and write from
         the first serial port.  It assumes that the BIOS has initialized the
         baud rate.

     Serial Bootselector /usr/mdec/mbr_com0_9600
         This has the same features as mbr_com0.  Additionally, it initializes
         the serial port to 9600 baud.

     The rest of this manual page will discuss the bootselecting versions 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, or -1 when it will wait for-
                        ever.

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

     The bootselector will output a menu of the bootmenu names for each parti-
     tion (as configured by fdisk(8)).  The user can then select the partition
     or drive to boot from via the keyboard.

     The numeric keys 1 upwards will initiate a startup from the corresponding
     partition.

     Function keys F1 through F8 (keys a through h for the serial versions)
     will boot from hard disks 0 through 7 (BIOS numbers 0x80 through 0x87).
     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.

     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 errors are detected:

     Code    Text message           Explanation
     1       No active partition    The MBR has a partition table without an
                                    active partition.
     2       Disk read error        There was an error reading the bootsector
                                    for the partition or drive selected.
     3       No operating system    The bootsector was loaded successfully,
                                    but it was not valid (i.e., the magic num-
                                    ber check failed, or it contained no
                                    code).
     L       Invalid CHS read       The boot partition cannot be read using a
                                    CHS read and the system BIOS doesn't sup-
                                    port LBA reads.
     ?                              Unknown key.

     The standard boot code will output the text message and stop.  It may be
     necessary to reset to the system to continue.

     The bootselect code will output 'Error <code>' and await further input.


SEE ALSO

     disklabel(8), fdisk(8), installboot(8), mbrlabel(8), x86/boot(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, 404 excluding the
     partition table and bootselect menu).

     The error messages are necessarily terse.

NetBSD 10.1                    February 17, 2017                   NetBSD 10.1