Application I/O Interface

Application I/O Interface

• I/O devices are of two types:

1. Block devices      2. Character devices

• Block device stores information in fixed size blocks. Operating system assigns address to each block. A Block device is one with which the driver communicates by sending entire blocks of data. Examples for block devices: hard disks, USB cameras, Disk-On-Key.

• A block device is a device (e.g., a disk) that can host a file system. In UNIX systems, a block device can only handle I/O operations that transfer one or more whole blocks, which are usually 512 bytes in length.

• Every block device driver must provide an interface to the buffer cache as well as at the normal file operations interface. Each block device is identified by major and minor numbers.

• Block devices are characterized by the capability to read and write blocks of data to and from random locations on an addressable medium. Examples of block devices include hard drives and USB Flash drives.

• Block and character devices are represented for user space applications as files than can be manipulated using the traditional file API.

• A character device is one with which the driver communicates by sending and receiving single characters. Examples for character devices es serial ports, parallel ports and sound cards.

• The only difference between a character device and a regular file is that, user can always move back and forth in the regular file, whereas most character devices are just data channels, which user can only access sequentially. User can perform mmap and lseek operation on the character device.

• Character devices can be thought of as serial streams of sequential data. Examples of character devices include serial ports and keyboards.

• Linux supports loadable device drivers, it is relatively easy to demonstrate a simple device driver skeleton. A device driver is a special kind of binary module. Unlike a stand-alone binary executable application, a device driver cannot simply be executed from a command prompt.

• Character device drivers are referenced through special files in the file system. This file is stored in the /dev folder. The command "Is -l" is used to check the folder. The character "c" in the file listing indicates the file is a char device. It also contains device major and minor numbers.

Difference between Block and Character Device

Network Device

• Network devices are deal differently from block and character devices. But these devices operate like I/O devices. User can not directly transfer data to network devices.

• Socket interface is used for communicating with network devices. Data can be put into the socket at one end, and read out sequentially at the other end. Sockets are normally full-duplex, allowing for bi-directional data transfer.

• UNIX uses pipes, FIFO, streams, queue and mailbox as a network device. Each network device is represented by a device data structure. The device data structure contains information about the device and the addresses of functions that allow the various supported network protocols to use the device's services.

• Network device drivers can be built into the Linux kernel. Each potential network device is represented by a device data structure within the network device list.

Clock and Timers

• Clocks are required in multiprogramming environment. Computers have hardware clocks and timers.

• Three types of time services are commonly needed in modern systems:

1. Current time of day

2. Elapsed time

3. Set a timer for particular event.

• Operating system used these function for timer applications. When user creates a file, operating system record it time and date.

• A Programmable Interrupt Timer (PIT) can be used to trigger operations and to measure elapsed time. User process uses timer by using operating system interface.

• Hardware clock is used to generate interrupt by operating system. On most systems the system clock is implemented by counting interrupts generated by the