MBR(8) (x86)

MBR(8) | System Manager's Manual (x86) | 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/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.
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 settings 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 forever.
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 partition (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 error 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 number 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 support 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.

February 17, 2017 | NetBSD 9.4