MBR(8)

NAME
mbr, bootselect - Master Boot Record bootcode

DESCRIPTION
A 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 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
    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).

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 usual.

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

Keys F5 through 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.

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.

DIAGNOSTIC
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 successfully, but it was not valid (i.e. the magic number check failed).

SEE ALSO
fdisk(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.