====== Device ======
A device file is an interface to a device driver.  
A device file is sometimes called a special file or a device special file.
The device file allows a program to interact with the device using standard input/output system calls.

Two types of device file:
* character - serial stream of characters
* block - fixed size blocks, random access, buffered, example: disk

Most device files are managed as part of a virtual file system mounted at /dev.

Device file is pointer, not container.

size
device numbers, major and minor
major driver
minor instance


full
null
random
urandom
zero


ls -li /dev | grep cdrom


ls -li /dev | grep tty


pts pseudo terminal slave
a type of tty


/dev/pts
character special file


xterm


bus 
        PCI, ISA, EISA




cpu clock, 200 MHz pulses, per second
system clock, time of day
time-sharing clock, interrupt cpu as reminder to switch users


chipset
        NorthBridge
        SouthBridge