1. Fall 2000M.B. Ibáñez Lecture 26 I/O Systems II

2. Fall 2000M.B. Ibáñez Application I/O Interface I/O system calls encapsulate device behaviors in generic classes We discuss structuring techniques and interfaces for the operating system that enable I/O devices to be treated in a standard, uniform way The approach involves abstraction, encapsulation, and software layering

3. Fall 2000M.B. Ibáñez Block-device interface The block device interface captures all the aspects necessary for accessing disk drivers and other block- oriented devices (disks, tapes) They store information in blocks that are usually fixed size, and transfers are made a block at a time The device understands commands such as read and write If it is a random-access device, it has a seek command to specify which block to transfer next

4. Fall 2000M.B. Ibáñez Character-stream interface The basic system calls in this interface enable an application to get or put one character On top of this interface, libraries can be built that offer line-at-a-time access, with buffering and editing services Convenient for –input devices such as keyboards, mice, and modems –output devices such as printers or audio boards

5. Fall 2000M.B. Ibáñez Network Devices Network I/O differ significantly from disk I/O One interface that is available in many operating systems is the network socket interface The system calls in the socket interface enable an application to –Create a socket –Connect a local socket to a remote address –Listen for any remote application to plug into the local socket –Send and receive packets over the connection

6. Fall 2000M.B. Ibáñez Clocks and Timers Most computers have hardware clocks and timers that provide three basic functions –Give the current time –Give the elapsed time –Set a timer to trigger operation O at time T

7. Fall 2000M.B. Ibáñez Blocking System Call When an application issues a blocking system call, the execution of the application is suspended The application is moved from the OS’s run queue to a wait queue After the system call completes, the application is moved back to the run queue, where it is eligible to resume execution, at which time it will receive the values returned by the system call

8. Fall 2000M.B. Ibáñez Non blocking (asynchronous) I/O Examples of non-blocking I/O –User interface that receives keyboard and mouse input while processing and displaying data on the screen –A video application that reads frames from a file on disk while simultaneously decompressing and displaying the output on the display An asynchronous call returns immediately, without waiting for the I/O to complete. The application continues to execute its code, and the completion of the I/O at some future time is communicated to the application

9. Fall 2000M.B. Ibáñez I/O scheduling To schedule a set of I/O requests means to determine a good order in which to execute them Scheduling can improve overall system performance, can share access fairly among processes, and can reduce the average waiting time for I/O to complete

10. Fall 2000M.B. Ibáñez Buffering A buffer is a memory area that stores data while they are transferred –between two devices –or between a device and an application Buffering is done for three reasons –To cope with a speech mismatch between the producer and the consumer of a data stream –To adapt between devices that have different data-transfer sizes –To support copy semantics for application I/O

11. Fall 2000M.B. Ibáñez No Buffering Operating SystemUser Process No buffering I/O Device In From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

12. Fall 2000M.B. Ibáñez Single Buffer Operating system assigns a buffer in main memory for an I/O request Block-oriented –input transfers made to buffer block moved to user space when needed –another block is moved into the buffer read ahead Operating SystemUser Process Single buffering I/O Device InMove From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

13. Fall 2000M.B. Ibáñez Single Buffer Block-oriented –user process can process one block of data while next block is read in –swapping can occur since input is taking place in system memory, not user memory –operating system keeps track of assignment of system buffers to user processes –output is accomplished by the user process writing a block to the buffer and later actually written out From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

14. Fall 2000M.B. Ibáñez Single Buffer Stream-oriented –used a line at time –user input from a terminal is one line at a time with carriage return signaling the end of the line –output to the terminal is one line at a time From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

15. Fall 2000M.B. Ibáñez Double Buffer Use two system buffers instead of one A process can transfer data to or from one buffer while the operating system empties or fills the other buffer I/O Device In Move Operating SystemUser Process Double buffering From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall

16. Fall 2000M.B. Ibáñez Circular Buffer More than two buffers are used Each individual buffer is one unit in a circular buffer Used when I/O operation must keep up with process I/O Device InMove Operating SystemUser Process Circular buffering.. From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall