Inside the kernel are functions for each of the devices the kernel is going to access. All the routines for a specific device are jointly referred to as the Device driver. Each device on the system has its own device driver. Eg: for devices such as a hard disk or terminal, the system needs to be able to open the device, write to the device, read from the device, and close the device, therefore, the respective drivers will contain the routines needed ..
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 and this special information is accomplished by Major and Minor numbers.
The major device numbers are used to distinguish between different devices and their controllers.The major number is actually the offset into the kernel's device driver table, which tells the kernel what kind of device it is.
The minor number tells the kernel special characteristics of the device to be accessed.
It is through this table that the routines are accessed that, in turn, access the physical hardware. Once the kernel has determined what kind of device to which it is talking, it determines the specific device, the specific location, or other characteristics of the device by means of the minor number.
The major number for the sd (SATA) driver is hard-coded at 8. The minor numbers have the format
(<unit>*16)+<part>) where,
unit- SATA drive number on the first controller either 0 or 1. so all sd devices has minor number less than 16.
part- Partition number, which can be anything from 1 to 20.
Which minor numbers you will be able to access will depend on how many partitions you have and what kind they are (extended, logical, etc.). The minor number of the device node that represents the whole disk is 0. This has the node name sda, whereas the other device nodes have a name equal to their minor number (/dev/sda2 has a minor number 2)
root@slackware:/dev]#fdisk -l
Disk /dev/sda: 48.1 GB, 48181477376 bytes
255 heads, 63 sectors/track, 5857 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x20d0a085
Device Boot Start End Blocks Id System
/dev/sda1 * 1 1824 14651248+ 83 Linux
/dev/sda2 1825 5857 32395072+ 5 Extended
/dev/sda5 1825 5654 30764443+ 83 Linux
/dev/sda6 5655 5857 1630566 82 Linux swap
From above we can conclude that :-
1. /dev/sda is the first controller and it's unit=0
2. Disk partitions are /dev/sda1, /dev/sda2, /dev/sda5, /dev/sda6.
Minor numbers are calculated as per formula -
(<unit>*16)+<part>):Substituting above we get,
(0*16)+0=0
(0*16)+1=1
(0*16)+2=2
(0*16)+5=5
(0*16)+6=6
In-case of another controller(unit=1), you would have minor numbers > 16.
root@slackware:/dev]#pwd
/dev
root@slackware:/dev]#ls -l sda*
brw-rw---- 1 root disk 8, 0 2012-03-19 13:06 sda
brw-rw---- 1 root disk 8, 1 2012-03-19 13:06 sda1
brw-rw---- 1 root disk 8, 2 2012-03-19 13:06 sda2
brw-rw---- 1 root disk 8, 5 2012-03-19 13:06 sda5
brw-rw---- 1 root disk 8, 6 2012-03-19 13:06 sda6
you could calculate Major and Minor according to the device info (Char/Block).
root@slackware:~]#cat /proc/devices
Character devices:
1 mem
2 pty
3 ttyp
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
6 lp
7 vcs
9 st
10 misc
13 input
14 sound
21 sg
29 fb
89 i2c
99 ppdev
116 alsa
128 ptm
136 pts
171 ieee1394
180 usb
189 usb_device
245 pcmcia
246 rtc
247 hidraw
248 usb_endpoint
249 usbmon
250 gdth
251 megaraid_sas_ioctl
252 megadev_legacy
253 aac
254 bsg
Block devices:
1 ramdisk
3 ide0
259 blkext
7 loop
8 sd
9 md
11 sr
22 ide1
65 sd
66 sd
67 sd
68 sd
69 sd
70 sd
71 sd
80 i2o_block
128 sd
129 sd
130 sd
131 sd
132 sd
133 sd
134 sd
135 sd
253 device-mapper
254 mdp
More information on "Devices" could be found in the below path.
root@slackware:~]#ls -l /usr/src/linux/Documentation/devices.txt
-rw-r--r-- 1 root root 118525 2009-07-02 23:41 /usr/src/linux/Documentation/devices.txt
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 and this special information is accomplished by Major and Minor numbers.
The major device numbers are used to distinguish between different devices and their controllers.The major number is actually the offset into the kernel's device driver table, which tells the kernel what kind of device it is.
The minor number tells the kernel special characteristics of the device to be accessed.
It is through this table that the routines are accessed that, in turn, access the physical hardware. Once the kernel has determined what kind of device to which it is talking, it determines the specific device, the specific location, or other characteristics of the device by means of the minor number.
The major number for the sd (SATA) driver is hard-coded at 8. The minor numbers have the format
(<unit>*16)+<part>) where,
unit- SATA drive number on the first controller either 0 or 1. so all sd devices has minor number less than 16.
part- Partition number, which can be anything from 1 to 20.
Which minor numbers you will be able to access will depend on how many partitions you have and what kind they are (extended, logical, etc.). The minor number of the device node that represents the whole disk is 0. This has the node name sda, whereas the other device nodes have a name equal to their minor number (/dev/sda2 has a minor number 2)
root@slackware:/dev]#fdisk -l
Disk /dev/sda: 48.1 GB, 48181477376 bytes
255 heads, 63 sectors/track, 5857 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x20d0a085
Device Boot Start End Blocks Id System
/dev/sda1 * 1 1824 14651248+ 83 Linux
/dev/sda2 1825 5857 32395072+ 5 Extended
/dev/sda5 1825 5654 30764443+ 83 Linux
/dev/sda6 5655 5857 1630566 82 Linux swap
From above we can conclude that :-
1. /dev/sda is the first controller and it's unit=0
2. Disk partitions are /dev/sda1, /dev/sda2, /dev/sda5, /dev/sda6.
Minor numbers are calculated as per formula -
(<unit>*16)+<part>):Substituting above we get,
(0*16)+0=0
(0*16)+1=1
(0*16)+2=2
(0*16)+5=5
(0*16)+6=6
In-case of another controller(unit=1), you would have minor numbers > 16.
root@slackware:/dev]#pwd
/dev
root@slackware:/dev]#ls -l sda*
brw-rw---- 1 root disk 8, 0 2012-03-19 13:06 sda
brw-rw---- 1 root disk 8, 1 2012-03-19 13:06 sda1
brw-rw---- 1 root disk 8, 2 2012-03-19 13:06 sda2
brw-rw---- 1 root disk 8, 5 2012-03-19 13:06 sda5
brw-rw---- 1 root disk 8, 6 2012-03-19 13:06 sda6
you could calculate Major and Minor according to the device info (Char/Block).
root@slackware:~]#cat /proc/devices
Character devices:
1 mem
2 pty
3 ttyp
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
6 lp
7 vcs
9 st
10 misc
13 input
14 sound
21 sg
29 fb
89 i2c
99 ppdev
116 alsa
128 ptm
136 pts
171 ieee1394
180 usb
189 usb_device
245 pcmcia
246 rtc
247 hidraw
248 usb_endpoint
249 usbmon
250 gdth
251 megaraid_sas_ioctl
252 megadev_legacy
253 aac
254 bsg
Block devices:
1 ramdisk
3 ide0
259 blkext
7 loop
8 sd
9 md
11 sr
22 ide1
65 sd
66 sd
67 sd
68 sd
69 sd
70 sd
71 sd
80 i2o_block
128 sd
129 sd
130 sd
131 sd
132 sd
133 sd
134 sd
135 sd
253 device-mapper
254 mdp
More information on "Devices" could be found in the below path.
root@slackware:~]#ls -l /usr/src/linux/Documentation/devices.txt
-rw-r--r-- 1 root root 118525 2009-07-02 23:41 /usr/src/linux/Documentation/devices.txt
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