https://en.wikipedia.org/wiki/Master_boot_record
https://wiki.archlinux.org/index.php/Master_Boot_Record
You can only have 4 primary partitions.
Each one can be either divided into logical any number of logical partitions partitions.
A primary partition that is split into logical partitions is called an extended partition.
Primary partitions get numbers from 1 to 4.
Logical partitions get numbers starting from 5.
You can visualize which partition is inside which disk with sudo lsblk -l.
TODO more common?
The MBR is the first 512 sector of the device found. It contains:
-
a small piece of code (446 bytes) called the primary boot loader.
This code will then be executed.
Most often, all it does it to load a second stage bootloader like GRUB.
-
the partition table (64 bytes) describing the primary and extended partitions.
-
2 bytes for MBR validation check.
It is a fixed magic number http://stackoverflow.com/questions/1125025/what-is-the-role-of-magic-number-in-boot-loading-in-linux, that can be used to:
- check if the device has a MBR
- check endianess
The MBR can only be at the start of a physical partition, not of a logical partition.
This is why on bootloader configurations you give /dev/sda, instead of /dev/sda1-4.