MBR(8) | System Manager's Manual (x86) | MBR(8) |
mbr
, bootselect
— Master Boot Record bootcode
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:
The rest of this manual page will discuss the bootselecting versions of the MBR. The configurable items of the bootselector are:
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.
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 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.
disklabel(8), fdisk(8), installboot(8), mbrlabel(8), x86/boot(8)
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 10.99 |