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Chapter 13: I/O Systems
Operating System Concepts – 8th Edition,
Silberschatz, Galvin and Gagne ©2009
Chapter 13: I/O Systems
 I/O Hardware
 Application I/O Interface
 Kernel I/O Subsystem
 Transforming I/O Requests to Hardware Operations
 STREAMS
 Performance
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Objectives
 Explore the structure of an operating system’s I/O subsystem
 Discuss the principles of I/O hardware and its complexity
 Provide details of the performance aspects of I/O hardware and software
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The Requirements of I/O
 So far in this course:
We have learned how to manage CPU, memory
 What about I/O?
 Without I/O, computers are useless (disembodied brains?)
 But… thousands of devices, each slightly different
 How can we standardize the interfaces to these devices?
 Devices unreliable: media failures and transmission errors
 How can we make them reliable???
 Devices unpredictable and/or slow
 How can we manage them if we don’t know what they will do or
how they will perform?

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I/O: Some operational parameters
 Byte/Block
Some devices provide single byte at a time (e.g. keyboard)
 Others provide whole blocks (e.g. disks, networks, etc)
 Sequential/Random
 Some devices must be accessed sequentially (e.g. tape)
 Others can be accessed randomly (e.g. disk, cd, etc.)
 Polling/Interrupts
 Some devices require continual monitoring
 Others generate interrupts when they need service

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I/O Hardware
 Incredible variety of I/O devices
 Common concepts

Port

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/O
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Modern I/O Systems
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A Typical PC Bus Structure
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Main components of Intel Chipset:
Pentium 4
 Northbridge:

Handles memory

Graphics
 Southbridge: I/O

PCI bus

Disk controllers

USB controllers

Audio

Serial I/O

Interrupt controller

Timers
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The Goal of the I/O Subsystem
 Provide Uniform Interfaces, Despite Wide Range of Different Devices

This code works on many different devices:
FILE fd = fopen(“/dev/something”,”rw”);
for (int i = 0; i < 10; i++) {
fprintf(fd,”Count %d\n”,i);
}
close(fd);

Why? Because code that controls devices (“device driver”) implements
standard interface.
 We will try to get a flavor for what is involved in actually controlling devices in
rest of lecture

Can only scratch surface!
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Device I/O Port Locations on PCs (partial)
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How does the processor actually talk to the device?
Processor Memory Bus
Bus
Adaptor
CPU
Interrupt
Controller
Other Devices
or Buses
Regular
Memory
Bus
Adaptor
Address+
Data
Interrupt Request
 CPU interacts with a Controller

Device
Controller
Bus
Hardware
Interface Controller
read
write
control
status
Registers
(port 0x20)
Addressable
Memory
and/or
Queues
Contains a set of registers that
can be read and written
Memory Mapped
 May contain memory for request
Region: 0x8f008020
queues or bit-mapped images
 Regardless of the complexity of the connections and buses, processor accesses
registers in two ways:
 I/O instructions: in/out instructions
 Example from the Intel architecture: out 0x21,AL
 Memory mapped I/O: load/store instructions
 Registers/memory appear in physical address space
 I/O accomplished with load and store instructions
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Want Standard Interfaces to Devices
 Block Devices: e.g. disk drives, tape drives, DVD-ROM

Access blocks of data
 Commands include open(), read(), write(), seek()

Raw I/O or file-system access
 Memory-mapped file access possible
 Character Devices: e.g. keyboards, mice, serial ports, some USB devices
 Single characters at a time
 Commands include get(), put()

Libraries layered on top allow line editing
 Network Devices: e.g. Ethernet, Wireless, Bluetooth
 Different enough from block/character to have own interface
 Unix and Windows include socket interface
 Separates network protocol from network operation
 Includes select() functionality

Usage: pipes, FIFOs, streams, queues, mailboxes
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Polling
 Determines state of device

command-ready

busy

Error
 Busy-wait cycle to wait for I/O from device
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Interrupts
 CPU Interrupt-request line triggered by I/O device
 Interrupt handler receives interrupts
 Maskable to ignore or delay some interrupts
 Interrupt vector to dispatch interrupt to correct handler

Based on priority

Some nonmaskable
 Interrupt mechanism also used for exceptions
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Interrupt-Driven I/O Cycle
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Intel Pentium Processor Event-Vector Table
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Example: Memory-Mapped Display Controller
 Memory-Mapped:

Hardware maps control registers and display
memory into physical address space
 Addresses set by hardware jumpers or
programming at boot time
 Simply writing to display memory (also
called the “frame buffer”) changes image on
screen
 Addr: 0x8000F000—0x8000FFFF
 Writing graphics description to commandqueue area
 Say enter a set of triangles that describe
some scene
 Addr: 0x80010000—0x8001FFFF
 Writing to the command register may cause
on-board graphics hardware to do
something
 Say render the above scene
 Addr: 0x0007F004
 Can protect with page tables
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0x80020000
0x80010000
Graphics
Command
Queue
Display
Memory
0x8000F000
0x0007F004 Command
0x0007F000 Status
Physical Address
Space
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Transfering Data To/From Controller
 Programmed I/O:

Each byte transferred via processor in/out or load/store

Pro: Simple hardware, easy to program

Con: Consumes processor cycles proportional to data size
 Direct Memory Access:

Give controller access to memory bus

Ask it to transfer data to/from memory directly
 Sample interaction with DMA controller (from book):
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Direct 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 memory
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Six Step Process to Perform DMA Transfer
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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

Memory-mapped file access possible
 Character devices include keyboards, mice, serial ports

Commands include get(), 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
 Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
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Clocks and Timers
 Provide current time, elapsed time, timer
 Programmable interval timer used for timings, periodic interrupts
 ioctl() (on UNIX) covers odd aspects of I/O such as clocks and timers
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How Does User Deal with Timing?
 Blocking Interface: “Wait”

When request data (e.g. read() system call), put process to sleep until
data is ready

When write data (e.g. write() system call), put process to sleep until
device is ready for data
 Non-blocking Interface: “Don’t Wait”

Returns quickly from read or write request with count of bytes successfully
transferred

Read may return nothing, write may write nothing
 Asynchronous Interface: “Tell Me Later”

When request data, take pointer to user’s buffer, return immediately; later
kernel fills buffer and notifies user

When send data, take pointer to user’s buffer, return immediately; later
kernel takes data and notifies user
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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
 Asynchronous - process runs while I/O executes

Difficult to use

I/O subsystem signals process when I/O completed
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Two I/O Methods
Synchronous
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Asynchronous
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Kernel 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”
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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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I/O Protection
 User process may accidentally or purposefully attempt to disrupt normal
operation via illegal I/O instructions

All I/O instructions defined to be privileged

I/O must be performed via system calls

Memory-mapped and I/O port memory locations must be protected
too
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Use of a System Call to Perform I/O
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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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STREAMS
 STREAM – a full-duplex communication channel between a user-level
process and a device in Unix System V and beyond
 A STREAM consists of:
- STREAM head interfaces with the user process
- driver end interfaces with the device
- zero or more STREAM modules between them.
 Each module contains a read queue and a write queue
 Message passing is used to communicate between queues
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The STREAMS Structure
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Performance
 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 stressful
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Intercomputer Communications
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Improving 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 throughput
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Device-Functionality Progression
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End of Chapter 13
Operating System Concepts – 8th Edition,
Silberschatz, Galvin and Gagne ©2009