Why do two devices have the same primary minor device number?

Question

I read "Linux device drivers 3rd edition" and found something that I cannot understand.

in chapter 3.2, the author said:

Traditionally, the primary number identifies the driver associated with the device. The lowest number is used by the kernel to determine which device it belongs to.

Then I tried "ls -l / dev" to look, I found something unusual:

brw-rw---- 1 root disk 1, 1 2011-08-23 23:52 ram1 brw-rw---- 1 root disk 1, 2 2011-08-23 23:52 ram2 brw-rw---- 1 root disk 1, 3 2011-08-23 23:52 ram3 brw-rw---- 1 root disk 1, 4 2011-08-23 23:52 ram4 brw-rw---- 1 root disk 1, 5 2011-08-23 23:52 ram5 ... crw-r----- 1 root kmem 1, 1 2011-08-23 23:52 mem crw-r----- 1 root kmem 1, 4 2011-08-23 23:52 port crw-rw-rw- 1 root root 1, 3 2011-08-23 23:52 null crw-rw-rw- 1 root root 1, 5 2011-08-23 23:52 zero 

All of these devices (ram1-ram5) have a clone with the same basic, but different name and type. I thought the author said that "the main number means the device class, and the lowest number means the device index. Therefore, Major-Minor identifies a unique device."

Now I am confused. Why can two devices have the same major? What are the device numbers?

Correct me if I am wrong .. Thanks in advance.

Answer 1

mem , port , null and zero are character devices (as evidenced c beginning of the list). ramN devices are block devices (hence b ). Major / minor numbers for block devices are independent of character devices and vice versa.