TANNENBAUM: 5, BIC & SHAW, SIBERSCHATZ: 12 INPUT/OUTPUT FROM DEVICE CLASSES TO DEVICES THEMES Wild disparity among i/o devices Fast CPU slow (even among the fastest) i/o devices What is the CPU to do when i/o is happening? How does CPU know when device is ready? How is data transferred to/from i/o device to RAM How to classify i/o devices in all their spendor? COMMON I/O DEVICES Notice the difference in data rates. Puts pressure on software to deal with data rates that vary by many orders of magnitude. HIERARCHICAL VIEW OF I/O SYSTEM NOTICE THE STRUCTURE Driver: often a standard interface (SATA, SCSI, USB...) to controller. Available to the o/s Device Controller/Adapter: Electronic Part Controls mechanical device For disk: convert serial bit stream into a block of bytes For monitor: reads bytes from memory, generates signals to modify polarization of appropriate pixels Without monitor controller: O/S developer would have to explicitly program the electric fields of all pixels THREE BASIC FUNCTIONS 1.Presents logical/abstract view of communication and storage devices by hiding details of physical devices 2.Allows efficient use of communication and storage devices 3.Supports convenient sharing of communication and storage devices WHERE I/O FITS Layer of software that resides between low-level hardware of individual devices and the higher level o/s subsystems (like the file system or virtual memory) that use the devices I/O subsystem never communicates with devices directly but only through device controllers TWO LAYERS OF I/O SOFTWARE Device Drivers Embody specific knowledge of devices they access Supplied by the manufacturer of the device New driver must be installed whenever a new device is added Device Independent Software Drivers can be abstracted into groups controlled by common software Block-oriented device management Stream (or character)-oriented management Network communications software BLOCK-DEVICE INTERFACE Best Example: Disk Storage devices that organize data into contiguous blocks of fixed size Support direct access to blocks through block number or address BLOCK DEVICE OPERATIONS Open: device is operational and prepares it for access Read: copies the contents of a specified block into a specified area of RAM Write: overwrites a specified block with data copied from a region of RAM Close: releases the device or decrements the number of processes sharing it. File-System uses the block-device interface to access the disk Virtual Memory accesses the block-device interface during a page fault THE STREAM DEVICE INTERFACE Best Example: Keyboard/Monitor Sometimes called character device. Controls devices that produce/consume streams of characters of arbitrary length Characters within a stream may not be addressed directly Stream devices must be used exclusively STREAM DEVICE OPERATIONS get: returns next character in stream put: appends character to output stream open/close: reserve/release a device Stream devices vary greatly (printer vs. a graphics monitor) so the interface needs an io_control that accepts different parameters to express device- specific functions NETWORK COMMUNICATION INTERFACE Best Example: Network Adapter Makes it possible for a computer to communicate with other computers and devices NETWORK INTERFACE OPERATIONS Socket: High-level abstraction for connection between two computers Two devices that wish to communicate create a socket bind to each others network address Use TCP/IP to handle low-level read/write Like so many other things invented by DOD in sixties and seventies DARPA, Vinton Cerf, Robert Kahn A Stanford Research Institute packet radio van, site of the first three-way internetworked transmission DIAGRAM OF FIRST INTERNETWORKED COMMUNICATION A FEW DEVICE CLASSES: TERMINALS Monitors: character-based or graphic Keyboards Pointing Devices (mouse, invented in the early 60s MONITORS a: character at a time displayed on screen at cursor position or interpreted as a control character In Action Here it is b: video ram memory holds copy of entire image. Each pixel is a separate cell in video ram. Since cells are randomly accessible, the image may be changed by modifying corresponding cells KEYBOARDS QWERTY keyboards date to the 2 nd half of the 19 th century From POV of i/o system, keyboards are similar to character-based monitors Pressing a key causes a character to be appended to a stream of characters in controller input buffer control characters operate on the buffer erase it delete the last character delete the last word (final project) OTHERS Pointer Devices (invented in the early sixties) Movement is detected by the hardware and translated into a byte stream I/O software retrieves byte stream from input buffer and makes it available to application softwarewhich determines the corresponding cursor movement or action resulting from button pressing Printers Impact: line, daisy wheel, dot-matrix non-impact: thermal, ink-jet, laser All are character devices: accept output in the form of a stream of control and printable characters. Stream is device specific and hidden inside device drivers DISKS Organized into concentric rings or tracks Tracks subdivided into sectors Each sector is a unit of information that can be read with a single operation read/write head positioned over the track containing the desired sector DISK SECTOR NUMBERING SCHEMES a.(track,sector) b.Sequential numbering d.Double Sided c. Track Skew Reading 17 and 18 requires going from track 0 to track 1. If 18 were adjacent to 17, will have passed by the time the r/w head gets to track 2. To fix the problem, offset the sequential numbering by one or more sectors. Here is track skew of 1 DISK CYLINDERS