Categories of I/O Devices

1
Categories of I/O Devices

• Human readable
• Communication with the user
• Printers
• Video display terminals
• Display
• Keyboard
• Mouse

• Machine readable
• Communication with electronic equipment
• Disk drives
• Sensors
• Controllers/Actors

• Communication
• Communication with remote devices
• Digital line drivers
• Modems

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Differences in I/O Devices

• Complexity of control
• Unit of transfer
• Data may be transferred as a stream of bytes for
  a terminal or in larger blocks for a disk
• Data representation
• Encoding schemes
• Error conditions
• Devices respond to errors differently

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Differences in I/O Devices

• Programmed I/O
• Process is busy-waiting for the operation to
  complete
• Interrupt-driven I/O
• I/O command is issued
• Processor continues executing instructions
• I/O module sends an interrupt when done
• Data Rate
• Human input can be very slow
• Machine communication should be fast

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Differences Data Rate
5
Operating System Design Issues

• Efficiency
• Most I/O devices extremely slow compared to main
  memory
• Use of multiprogramming allows for some processes
  to be waiting on I/O while another process
  executes
• I/O cannot keep up with processor speed
• Swapping is used to bring in additional Ready
  processes which is an I/O operation

6
Operating System Design Issues

• Generality
• Desirable to handle all I/O devices in a uniform
  manner
• Hide most of the details of device I/O in
  lower-level routines so that processes and upper
  levels see devices in general terms such as read,
  write, open, close, lock, unlock

7
Data Transfer Scheme

• Block-oriented
• Information is stored in fixed sized blocks
• Transfers are made a block at a time
• Used for disks and tapes
• Stream-oriented
• Transfer information as a stream of bytes
• Used for terminals, printers, communication
  ports, mouse, and most other devices that are not
  secondary storage

8
I/O Buffering

• Reasons for buffering
• Processes must wait for I/O to complete before
  proceeding (you cant do anything useful anyway)
• Adjust speed differences between information
  source and use
• Example Printer buffer
• You dont want your computer to sit and wait in
  order to transfer data until there is no other
  job ahead in the queue and the data could go
  directly to printer
• Instead printer buffers data

9
Techniques for Performing I/O

• Direct Memory Access (DMA)
• DMA module controls exchange of data between main
  memory and the I/O device
• Processor interrupted only after entire block has
  been transferred
• CPU independent, CPU can do something else while
  I/O data transfer to memory happens

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Direct Memory Access

• Takes control of the system form the CPU to
  transfer data to and from memory over the system
  bus
• Cycle stealing is used to transfer data on the
  system bus
• The instruction cycle is suspended so data can
  be transferred
• The CPU pauses one bus cycle
• No interrupts occur, no process swapping
• Do not save context

11
Communication Inputs

• I/O Input Output
• Output CPU initiated
• Input Initiated by device
• Creates an interrupt to inform CPU of input
• Typical Example Communication in Client Server
  environment

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Examples of Client Server

• Application-level protocols provide high-level
  services
• DNS
• Electronic mail
• TELNET Remote login
• FTP
• HTTP World Wide Web
• All of these applications use client-server
  architecture

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Client Server Architecture
Client Computer
Server Computer
Client Application
Server Application
Client Request
Server Response
Network
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Client Server Paradigm

• Server application is listener''
• Waits for incoming message
• Performs service
• Returns results
• Client application establishes connection
• Sends message to server
• Waits for return message

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Issues

• Identification of Server computer
• Identification of Client computer
• Identification of Server Application
• Identification of Client Application
• How do you ensure that both application
  understand each other?
• Answer IP address, Port number , TCP

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Client

• Arbitrary application program
• Becomes client when network service is needed
• Also performs other computations
• Invoked directly by user
• Runs locally on user's computer
• Initiates contact with server
• Can access multiple services (one at a time)
• Does not require special hardware or
  sophisticated operating system

17
Server

• Special purpose application dedicated to
  providing network service
• Starts at system initialization time
• Runs on a remote computer (usually centralized,
  shared computer)
• Waits for service requests from clients loops to
  wait for next request
• Will accept requests from arbitrary clients
  provides one service to each client
• Requires powerful hardware and sophisticated
  operating system

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Server Class Computer

• Shared, centralized computers that run many
  server applications called servers''
• More precisely, the applications are the
  servers'' and the computer is a server-class
  computer''
• Servers can run on very simple computers...

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Message exchange

• Typically, client and server exchange messages
• Client sends request, perhaps with data
• Server send response, perhaps with data
• Client may send multiple requests server sends
  multiple responses
• Server may send multiple response - imagine video
  feed

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Message Protocols TCP

• Standards for messages that ensure that the
  server can understand the client independent of
  implementation

21
Multi Server and Client

• Multiple clients share same server
• One server class computer runs multiple server
  applications and types (eureka)
• Listener organizes

22
Service Identification

• Each service gets a unique identifier both
  client and server use that identifier
• Server registers with local protocol software
  under the identifier
• Client contacts protocol software for session
  under that identifier
• Example - TCP uses protocol port numbers as
  identifiers
• Server registers under port number for service
• Client requests session with port number for
  service
• How does client knows the port number on the
  server computer?

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Multiple servers for one service

• Responding to a client request may require
  significant time
• Other clients must wait while earlier requests
  are satisfied
• Multiple servers can handle requests
  concurrently, completing shorter requests without
  waiting for longer requests

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Master-slave servers

• One way to run concurrent servers is to
  dynamically create server processes for each
  client
• Master server accepts incoming requests and
  starts slave server for each client
• Slave handles subsequent requests from its client
  
• Master server then waits for next request

25
Examples of server running on eureka

• Ps u root
• Mountd
• In.telnetd
• Inetd
• Listen
• Program that does the listening for all server
  and starts a new serving program

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Review Interrupts

• External devices (network card) creates
  interrupts that are first dealt with by an
  interrupt controller (IC)
• IC sends highest priority interrupt to CPU and
  puts into the interrupt register the PC
  associated with the code that can deal with this
  type of interrupt
• Listener decides what to do
• Start new server
• Start running server who was the recipient of
  incoming message

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Selecting from multiple servers

• How do incoming messages get delivered to the
  correct server?
• Each transport session has two unique identifiers
  
• (IP address, port number) on server
• (IP address, port number) on client
• No two clients on one computer can use same
  source port
• Thus, client endpoints are unique, and server
  computer protocol software can deliver messages
  to correct server process