InputOutput

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Input/Output

• Two main jobs of a computer are
• Processing
• Compute an answer matrix multiplication, FFT
• ? OS must control the central processing unit,
  the CPU
• Process management (creation/termination),
  process scheduling, process synchronization,
  memory management
• Input/Output (I/O)
• Get user input from the keyboard -- Keyboard
• Display the chars that the user types on the
  screen -- Display
• Read/Save a file from/to the disk Floppy, hard
  disk, CD-ROM
• Download a Web page using the Web Browser
  Network Card (NIC)
• ? OS must control all I/O devices
• Done by the I/O subsystem of the OS

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Typical PC Arch. OS Kernel Struc.
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I/O Subsystem

• I/O Subsystem Functions
• Provide OS-Device Interaction
• Issue commands to the devices, catch interrupts,
  handle errors
• Provide OS-Application Process Interaction
• Export an easy-to-use API to the applications
• Open, read, write, close

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I/O Subsystem

• I/O Subsystem can roughly be divided into 2
  layers
• A device dependent part, called the device
  driver, which implements the OS-device
  interaction part
• A device independent part, which implements the
  OS-User Application Process interaction part

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I/O Subsystem

• Device drivers implement the OS-device
  interaction
• Interacts with the device controller using the
  controller interface (Hardware Controller API)
• Exports an interface to the Device Independent
  Software Layer above it (Device Driver API)

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Device Drivers (more)

• Interacting with the device controller
  Controller API
• Driver knows the specifics of the device
• What registers does the device controller have?
• How to program these control registers and access
  data registers etc.
• Driver interacts with the device controller to
  perform I/O
• Issue commands to the controller to perform I/O
• Catch device interrupts
• Handle I/O device errors

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Device Independent Software

• Sits on top of the device drivers
• Uses the device driver API to perform actual I/O
• Exports a system-call API for application
  processes to perform I/O from the user space

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Device Driver API

• Interacting with the Device Independent Software
  layer Device Driver API
• Lots of I/O devices!
• We can define an interface for each type of
  device?
• Too many interfaces.
• Solution Classify I/O devices into one of a
  small number of categories and define an
  interface for each group.
• Block devices - Disks
• Those that store info in fixed sized blocks, each
  with its own address.
• It is possible to read/write each block
  independently of each other
• Character devices - keyboards, printers, NIC,
  mouse,
• Those that deliver or accept a stream of chars
  without regard to any block structure.
• Not addressable and does not have a seek operation

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Device Driver API

• Most OSs define
• a standard interface that all block device
  drivers must support
• a standard interface that all character device
  drivers must support
• These interfaces consists of a number of
  functions that the rest of the OS can call to get
  the driver to do work for it
• Read a block, write a char etc.
• A standard interface that all network device
  drivers must support
• Transmit a packet, receive a packet
• Since each OS defines its own device driver API,
  a new device driver must be implemented for each
  OS

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I/O System Call API -- Issues

• Device Naming/Protection How does an
  application name an I/O device such as a disk or
  keyboard?
• Assign symbolic names for each device and have
  DIS to map symbolic names onto proper driver
• /dev/disk0, /dev/tty0 Unix, Linux (part of the
  file system)
• C, D, COM1 -- Windows (separate name space)
• Symbolic names uniquely identify the device
  within the OS
• /dev/disk0 uniquely specifies the inode for a
  special file, and this inode contains the major
  device, which is used to locate the appropriate
  driver.
• The inode also contains the minor device , which
  is passed as a parameter to the driver in order
  to specify the unit to be read/written
• Setting access rights to different users?
• Simply set the appropriate file protection
  rights -- Unix

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I/O System Call API -- Issues

• I/O Call Semantics
• Blocking (Synchronous) I/O Calls
• process suspended until I/O completed
• Easy to use and understand
• Insufficient for some needs
• E.g., if the process is single threaded
• Non-Blocking (Asynchronous) I/O Calls
• process runs while I/O executes
• Difficult to use
• I/O subsystem signals process when I/O completed

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DIS Services

• Scheduling
• Some I/O request ordering via per-device queue
• Disk I/O (read/write) scheduling later
• Buffering - store data in memory while
  transferring between devices
• E.g., Network packets arriving for a process
  asynchronously
• Caching - fast memory holding copy of data
• Key to performance Keep the recently accessed
  disk blocks for faster read/write operations
  called the disk I/O cache

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DIS Services

• Keeping Track of User Process I/O State
• Kernel keeps state info for processes, including
  open file tables, network connections, character
  device state

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User-level I/O Software

• Typically a user-level I/O library is first used
  by the application programs. Its functions are
• Format I/O printf, fprintf, fopen,
• Buffering May also buffer data before making
  system calls to improve performance

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Input/Output Summary

• Handling of an I/O requests and list of each
  layer with their functionality

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Layers of I/O Subsystem in Linux
Process 2
Process 1
Process N
User Mode
Kernel Mode
Virtual File System
ext2fs
ntfs
fat32
proc
Buffer Cache
File System
Device Drivers