Chapter 13: I/O Systems

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Chapter 13 I/O Systems
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Chapter 13 I/O Systems

• I/O Hardware
• Application I/O Interface
• Kernel I/O Subsystem
• Transforming I/O Requests to Hardware Operations

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Objectives

• Explore the structure of an operating systems
  I/O subsystem
• Discuss the principles of I/O hardware and its
  complexity

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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

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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

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A Typical PC Bus Structure
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Device I/O Port Locations on PCs (partial)
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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

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Interrupt-Driven I/O Cycle
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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

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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

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A Kernel I/O Structure
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Characteristics of I/O Devices
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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

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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

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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

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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

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Two I/O Methods
Synchronous
Asynchronous
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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 applications buffer

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Device-status Table
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Sun Enterprise 6000 Device-Transfer Rates
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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

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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

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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

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UNIX I/O Kernel Structure
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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

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Life Cycle of An I/O Request
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End of Chapter 13