1. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 13: I/O Systems

2. 13.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 13: I/O Systems I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations STREAMS Performance

3. 13.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Objectives Explore the structure of an operating system’s I/O subsystem Discuss the principles of I/O hardware and its complexity Provide details of the performance aspects of I/O hardware and software

4. 13.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The Requirements of I/O So far in this course: We have learned how to manage CPU, memory What about I/O? Without I/O, computers are useless (disembodied brains?) But… thousands of devices, each slightly different  How can we standardize the interfaces to these devices? Devices unreliable: media failures and transmission errors  How can we make them reliable??? Devices unpredictable and/or slow  How can we manage them if we don’t know what they will do or how they will perform?

5. 13.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition I/O: Some operational parameters Byte/Block Some devices provide single byte at a time (e.g. keyboard) Others provide whole blocks (e.g. disks, networks, etc) Sequential/Random Some devices must be accessed sequentially (e.g. tape) Others can be accessed randomly (e.g. disk, cd, etc.) Polling/Interrupts Some devices require continual monitoring Others generate interrupts when they need service

6. 13.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition I/O Hardware Incredible variety of I/O devices Common concepts Port Bus (daisy chain or shared direct access) Controller (host adapter) I/O instructions control devices Devices have addresses, used by Direct I/O instructions Memory-mapped I/O

7. 13.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Modern I/O Systems

8. 13.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical PC Bus Structure

9. 13.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Main components of Intel Chipset: Pentium 4 Northbridge: Handles memory Graphics Southbridge: I/O PCI bus Disk controllers USB controllers Audio Serial I/O Interrupt controller Timers

10. 13.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The Goal of the I/O Subsystem Provide Uniform Interfaces, Despite Wide Range of Different Devices This code works on many different devices: FILE fd = fopen(“/dev/something”,”rw”); for (int i = 0; i < 10; i++) { fprintf(fd,”Count %d\n”,i); } close(fd); Why? Because code that controls devices (“device driver”) implements standard interface. We will try to get a flavor for what is involved in actually controlling devices in rest of lecture Can only scratch surface!

11. 13.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Device I/O Port Locations on PCs (partial)

12. 13.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Device Controller read write control status Addressable Memory and/or Queues Registers (port 0x20) Hardware Controller Memory Mapped Region: 0x8f008020 Bus Interface How does the processor actually talk to the device? CPU interacts with a Controller Contains a set of registers that can be read and written May contain memory for request queues or bit-mapped images Regardless of the complexity of the connections and buses, processor accesses registers in two ways: I/O instructions: in/out instructions  Example from the Intel architecture: out 0x21,AL Memory mapped I/O: load/store instructions  Registers/memory appear in physical address space  I/O accomplished with load and store instructions Address+ Data Interrupt Request Processor Memory Bus CPU Regular Memory Interrupt Controller Bus Adaptor Bus Adaptor Other Devices or Buses

13. 13.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Want Standard Interfaces to Devices Block Devices: e.g. disk drives, tape drives, DVD-ROM Access blocks of data Commands include open(), read(), write(), seek() Raw I/O or file-system access Memory-mapped file access possible Character Devices: e.g. keyboards, mice, serial ports, some USB devices Single characters at a time Commands include get(), put() Libraries layered on top allow line editing Network Devices: e.g. Ethernet, Wireless, Bluetooth Different enough from block/character to have own interface Unix and Windows include socket interface  Separates network protocol from network operation  Includes select() functionality Usage: pipes, FIFOs, streams, queues, mailboxes

14. 13.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Polling Determines state of device command-ready busy Error Busy-wait cycle to wait for I/O from device

15. 13.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Interrupts CPU Interrupt-request line triggered by I/O device Interrupt handler receives interrupts Maskable to ignore or delay some interrupts Interrupt vector to dispatch interrupt to correct handler Based on priority Some nonmaskable Interrupt mechanism also used for exceptions

16. 13.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Interrupt-Driven I/O Cycle

17. 13.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Intel Pentium Processor Event-Vector Table

18. 13.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example: Memory-Mapped Display Controller Memory-Mapped: Hardware maps control registers and display memory into physical address space  Addresses set by hardware jumpers or programming at boot time Simply writing to display memory (also called the “frame buffer”) changes image on screen  Addr: 0x8000F000—0x8000FFFF Writing graphics description to command- queue area  Say enter a set of triangles that describe some scene  Addr: 0x80010000—0x8001FFFF Writing to the command register may cause on-board graphics hardware to do something  Say render the above scene  Addr: 0x0007F004 Can protect with page tables Display Memory 0x8000F000 0x80010000 Physical Address Space Status 0x0007F000 Command 0x0007F004 Graphics Command Queue 0x80020000

19. 13.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Transfering Data To/From Controller Programmed I/O: Each byte transferred via processor in/out or load/store Pro: Simple hardware, easy to program Con: Consumes processor cycles proportional to data size Direct Memory Access: Give controller access to memory bus Ask it to transfer data to/from memory directly Sample interaction with DMA controller (from book):

20. 13.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Direct Memory Access Used to avoid programmed I/O for large data movement Requires DMA controller Bypasses CPU to transfer data directly between I/O device and memory

21. 13.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Six Step Process to Perform DMA Transfer

22. 13.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

23. 13.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Kernel I/O Structure

24. 13.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Characteristics of I/O Devices

25. 13.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Block and Character Devices Block devices include disk drives Commands include read, write, seek Raw I/O or file-system access Memory-mapped file access possible Character devices include keyboards, mice, serial ports Commands include get(), put() Libraries layered on top allow line editing

26. 13.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

27. 13.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Clocks and Timers Provide current time, elapsed time, timer Programmable interval timer used for timings, periodic interrupts ioctl() (on UNIX) covers odd aspects of I/O such as clocks and timers

28. 13.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition How Does User Deal with Timing? Blocking Interface: “Wait” When request data (e.g. read() system call), put process to sleep until data is ready When write data (e.g. write() system call), put process to sleep until device is ready for data Non-blocking Interface: “Don’t Wait” Returns quickly from read or write request with count of bytes successfully transferred Read may return nothing, write may write nothing Asynchronous Interface: “Tell Me Later” When request data, take pointer to user’s buffer, return immediately; later kernel fills buffer and notifies user When send data, take pointer to user’s buffer, return immediately; later kernel takes data and notifies user

29. 13.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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 Asynchronous - process runs while I/O executes Difficult to use I/O subsystem signals process when I/O completed

30. 13.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two I/O Methods Synchronous Asynchronous

31. 13.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Kernel I/O Subsystem Scheduling Some I/O request ordering via per-device queue Some OSs try fairness Buffering - store data in memory while transferring between devices To cope with device speed mismatch To cope with device transfer size mismatch To maintain “copy semantics”

32. 13.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Device-status Table

33. 13.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Sun Enterprise 6000 Device-Transfer Rates

34. 13.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

35. 13.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

36. 13.36 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition I/O Protection User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions All I/O instructions defined to be privileged I/O must be performed via system calls  Memory-mapped and I/O port memory locations must be protected too

37. 13.37 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Use of a System Call to Perform I/O

38. 13.38 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

39. 13.39 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition UNIX I/O Kernel Structure

40. 13.40 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition 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

41. 13.41 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Life Cycle of An I/O Request

42. 13.42 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition STREAMS STREAM – a full-duplex communication channel between a user-level process and a device in Unix System V and beyond A STREAM consists of: - STREAM head interfaces with the user process - driver end interfaces with the device - zero or more STREAM modules between them. Each module contains a read queue and a write queue Message passing is used to communicate between queues

43. 13.43 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The STREAMS Structure

44. 13.44 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Performance I/O a major factor in system performance: Demands CPU to execute device driver, kernel I/O code Context switches due to interrupts Data copying Network traffic especially stressful

45. 13.45 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Intercomputer Communications

46. 13.46 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Improving Performance Reduce number of context switches Reduce data copying Reduce interrupts by using large transfers, smart controllers, polling Use DMA Balance CPU, memory, bus, and I/O performance for highest throughput

47. 13.47 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Device-Functionality Progression

48. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 13