Major and Minor numbers

One of the basic features of the Linux kernel is that it abstracts the handling of devices. All hardware devices look like regular files. They can be opened, closed, read and written using the same, standart system calls that are used to manipulate files.

Every device in the system is represented by a file. For block (disk) and character devices, these device files are created by the mknod command and they describe the device using major and minor device numbers.

The kernel needs to be told how to access the device. Not only does the kernel need to be told what kind of device is being accessed but also any special information, such as the partition number if it's a hard disk or density if it's a floppy, for example. This is accomplished by the major number and minor number of that device.

All devices controlled by the same device driver have a common major device number. The minor device numbers are used to distinguish between different devices and their controllers. Linux maps the device special file passed in system calls (say to mount a file system on a block device) to the device's device driver using the major device number and a number of system tables, for example the character device table, chrdevs. The major number is actually the offset into the kernel's device driver table, which tells the kernel what kind of device it is (whether it is a hard disk or a serial terminal). The minor number tells the kernel special characteristics of the device to be accessed. For example, the second hard disk has a different minor number than the first. The COM1 port has a different minor number than the COM2 port, each partition on the primary IDE disk has a different minor device number, and so forth. So, for example, /dev/hda2, the second partition of the primary IDE disk has a major number of 3 and a minor number of 2.


Reference: The Linux tutorial