1Chapter 12: I/O Systems I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware OperationsI/O Hardware Incredible variety of I/O devices Common concepts Port (typically status, control, data-in, data-out) 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/O2A Typical PC Bus StructureDevice I/O Port Locations on PCs (partial)3Device-Functionality ProgressionPolling Determines state of device  command-ready busy Error Busy-wait cycle to wait for I/O from device It is a loop reading the status register over and over again A more efficient approach is through interrupts Let the host be notified by the device4Interrupts CPU Interrupt request line triggered by I/O device Interrupt handler receives interrupts Maskable to ignore or delay some interrupts Can be turned off by CPU Interrupt vector (contains the addresses of interrupt handlers) to dispatch interrupt to correct handler Based on priority Some non-maskable (e.g., reserved for unrecoverable memory memory errors) Interrupt mechanism also used for exceptionsInterrupt-Driven I/O Cycle5Intel Pentium Processor Event-Vector TableDirect 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 memory6Six Step Process to Perform DMA TransferApplication 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 only7A Kernel I/O StructureCharacteristics of I/O Devices8Block 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 editingNetwork Devices Varying enough from block and character to have own interface Unix and Windows NT/9i/2000 include socket interface Separates network protocol from network operation Includes select functionality Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)9Blocking 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 completedKernel 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”10Kernel 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 deadlockError 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 reports11Kernel 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” blocksI/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 process12Life Cycle of An I/O RequestPerformance 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 stressful13Improving 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