I/o systems

I/O Systems

Bajardi: Baxtiyorov Shahriyor

Tekshirdi: Atoyev Suhrob

Overview

• Management of I/O devices is a very important part of the operating system - so important and so varied that entire I/O subsystems are devoted to its operation. ( Consider the range of devices on a modern computer, from mice, keyboards, disk drives, display adapters, USB devices, network connections, audio I/O, printers, special devices for the handicapped, and many special-purpose peripherals. )
• I/O Subsystems must contend with two ( conflicting? ) trends: (1) The gravitation towards standard interfaces for a wide range of devices, making it easier to add newly developed devices to existing systems, and (2) the development of entirely new types of devices, for which the existing standard interfaces are not always easy to apply.
• Device drivers are modules that can be plugged into an OS to handle a particular device or category of similar devices.

I/O Hardware

• I/O devices can be roughly categorized as storage, communications, user-interface, and other
• Devices communicate with the computer via signals sent over wires or through the air.
• Devices connect with the computer via ports, e.g. a serial or parallel port.
• A common set of wires connecting multiple devices is termed a bus.

Polling

• One simple means of device handshaking involves polling:
• The host repeatedly checks the busy bit on the device until it becomes clear.
• The host writes a byte of data into the data-out register, and sets the write bit in the command register ( in either order. )
• The host sets the command ready bit in the command register to notify the device of the pending command.
• When the device controller sees the command-ready bit set, it first sets the busy bit.
• Then the device controller reads the command register, sees the write bit set, reads the byte of data from the data-out register, and outputs the byte of data.
• The device controller then clears the error bit in the status register, the command-ready bit, and finally clears the busy bit, signaling the completion of the operation.
• Polling can be very fast and efficient, if both the device and the controller are fast and if there is significant data to transfer. It becomes inefficient, however, if the host must wait a long time in the busy loop waiting for the device, or if frequent checks need to be made for data that is infrequently there.