1. Chapter 13: I/O Systems

2. 13.2 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Chapter 13: I/O Systems I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations

3. 13.3 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Objectives Explore the structure of an operating system’s I/O subsystem Discuss the principles of I/O hardware and its complexity

4. 13.4 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Overview The control of devices connected to the computer is a major concern of operating-system designers. I/O devices vary so widely in their function and speed, varied methods are needed to control them. Two conflicting trends of I/O-device technology: Increasing standardization of software and hardware interfaces An increasingly broad variety of I/O devices. To encapsulate the details and oddities of different devices, the kernel of an operating system is structured to use device-driver modules The device drivers present a uniform device-access interface to the I/O subsystem

5. 13.5 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 I/O Hardware Incredible variety of I/O devices Common concepts Port: a connection point Bus: a set of wires and a rigidly defined protocol that specifies a set of messages that can be sent on the wires Controller: a collection of electronics that can operate a port, a bus, or a device. I/O instructions control devices The controller has one or more registers for data and control signals. The processor communicates with the controller by reading or writing bit patterns in these registers How can the processor give commands and data to a controller to accomplish an I/O transfer? Direct I/O instructions Memory-mapped I/O

6. 13.6 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 A Typical PC Bus Structure

7. 13.7 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Device I/O Port Locations on PCs (partial)

8. 13.8 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Polling Determines state of device command-ready indicates command is available for the controller to execute Busy indicates the device is busy working Error indicates there is something wrong Basic handshaking notion: The host repeatedly reads the busy bit until it is clear The host sets the writes bit in the command register and writes a byte into the data-out register The host sets the command-ready bit When the controller notices that the command-ready bit is set, it sets the busy bit The controller reads the command register and sees the write command. It reads the data-out register to get the byte and does the I/O to the device. The controller clears the command-ready bit, clears the error bit in the status register to indicate that the device I/O succeeded, and clears the busy bit to indicate that it is finished

9. 13.9 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Interrupt-Driven I/O Cycle

10. 13.10 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Interrupts The hardware mechanism that enables a device to notify the CPU is called an interrupt Interrupt handler receives interrupts Most CPU has two Interrupt-request line triggered by I/O device: nonmaskable and maskable Nonmaskable is reserved for event such as unrecoverable memory erros Maskable to ignore or delay some interrupts

11. 13.11 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Application I/O Interface I/O system calls encapsulate device behaviors in generic classes Device-driver layer hides differences among I/O controllers from kernel Devices vary in many dimensions Character-stream or block Sequential or random-access Sharable or dedicated Speed of operation read-write, read only, or write only

12. 13.12 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 A Kernel I/O Structure

13. 13.13 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Characteristics of I/O Devices

14. 13.14 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Block and Character Devices Block devices include disk drives Commands include read, write, seek Raw I/O or file-system access for OS and database- management systems Memory-mapped file access possible Character devices include keyboards, mice, serial ports Commands include get and put Libraries layered on top allow line editing

15. 13.15 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Network Devices Varying enough from block and character to have own interface Unix and Windows NT/9x/2000 include socket interface Separates network protocol from network operation Includes select functionality: returns information about which sockets have a packet waiting to be received and which sockets have room to accept a packet to be sent

16. 13.16 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Clocks and Timers Provide current time, elapsed time, timer to trigger a certain operation at time T Programmable interval timer used for timings, periodic interrupts

17. 13.17 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Blocking and Nonblocking I/O Blocking - process suspended until I/O completed Easy to use and understand Insufficient for some needs Nonblocking - I/O call returns as much as available User interface, data copy (buffered I/O) Implemented via multi-threading Returns quickly with count of bytes read or written

18. 13.18 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Two I/O Methods Synchronous Asynchronous

19. 13.19 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Kernel I/O Subsystem Scheduling Some I/O request ordering via per-device queue: a waiting queue of request for each device Some OSs try fairness: rearrange the order of the queue to improve the overall system efficiency and the average response time Buffering - store data in memory while transferring between devices or between a device and an application To cope with device speed mismatch To cope with device transfer size mismatch To maintain “copy semantics”: the version of the data written to disk is guaranteed to be the version at the time of the application system call, independent of any subsequent changes in the application’s buffer

20. 13.20 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Device-status Table

21. 13.21 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Sun Enterprise 6000 Device-Transfer Rates

22. 13.22 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Kernel I/O Subsystem Caching - fast memory holding copy of data Always just a copy Key to performance Spooling - hold output for a device If device can serve only one request at a time i.e., Printing Device reservation - provides exclusive access to a device System calls for allocation and deallocation Watch out for deadlock

23. 13.23 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Error Handling OS can recover from disk read, device unavailable, transient write failures Most return an error number or code when I/O request fails System error logs hold problem reports

24. 13.24 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Kernel Data Structures Kernel keeps state info for I/O components, including open file tables, network connections, character device state Many, many complex data structures to track buffers, memory allocation, “dirty” blocks Some use object-oriented methods and message passing to implement I/O

25. 13.25 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 UNIX I/O Kernel Structure

26. 13.26 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 I/O Requests to Hardware Operations Consider reading a file from disk for a process: Determine device holding file Translate name to device representation Physically read data from disk into buffer Make data available to requesting process Return control to process

27. 13.27 Silberschatz, Galvin and Gagne ©2005 Operating System Concepts – 7 th Edition, Jan 2, 2005 Life Cycle of An I/O Request

28. End of Chapter 13