Transcript Categories of I/O Devices - NYU Stern School of Business

Categories of I/O Devices
• Human readable
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• Machine readable
Communication with the user
Printers
Video display terminals
Display
Keyboard
Mouse
– Communication with
electronic equipment
– Disk drives
– Sensors
– Controllers/Actors
• Communication
– Communication with remote devices
– Digital line drivers
– Modems
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
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
Differences: Data Rate
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
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 lowerlevel routines so that processes and upper levels
see devices in general terms such as read, write,
open, close, lock, unlock
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
I/O Buffering
• Reasons for buffering
– Processes must wait for I/O to complete before
proceeding (you can’t do anything useful anyway)
• Adjust speed differences between
information source and use
• Example: Printer buffer
– You don’t 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
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
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
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
Examples of Client Server
• Application-level protocols provide high-level
services
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DNS
Electronic mail
TELNET: Remote login
FTP
HTTP World Wide Web
• All of these applications use client-server
architecture
Client Server Architecture
Client Request
Client
Application
Server Response
Client Computer
Network
Server
Application
Server Computer
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
Issues
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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
Client
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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
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
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...
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
Message Protocols TCP
• Standards for messages that ensure that the
server can understand the client independent
of implementation
Multi Server and Client
• Multiple clients share same server
• One server class computer runs multiple
server applications and types (eureka)
• Listener organizes
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?
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
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
Examples of server running on
eureka
• Ps –u root
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Mountd
In.telnetd
Inetd
Listen:
• Program that does the listening for all server and
starts a new serving program
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
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