Chapter 13: I/O Systems
Operating System Concepts with Java – 7th Edition, Nov 15, 2006
Silberschatz, Galvin and Gagne ©2007
Chapter 13: I/O Systems
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I/O Hardware
Explicit I/O Instructions vs. Memory-Mapped I/O
Polling vs. Interrupts
Programmed I/O vs. DMA
Device Drivers
Application I/O Interface
Kernel I/O Subsystem
Transforming I/O Requests to Hardware Operations
Streams
Performance
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Objectives
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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
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Without I/O, computers are not very useful
 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 utilize them if we don’t know what they will do
or how they will perform?
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Some operational parameters:
 Byte/Block
 Some devices provide single byte at a time (e.g. keyboard)
 Others provide whole blocks (e.g. disks, tapes, 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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The Goal of the I/O Subsystem
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Provide Uniform Interfaces, Despite Wide Range of Different Devices
 This code works on many different devices:
int fd = open(“/dev/something”);
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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Modern I/O Systems
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I/O Devices
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Device Rates vary over
many orders of magnitude
 System better be able
to handle this wide
range
 Better not have high
overhead/byte for fast
devices!
 Better not waste time
waiting for slow
devices
Some typical device,
network, and bus data
rates.
Table from Tanenbaum, Modern Operating Systems
3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved.
0-13-6006639
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Intel Chipset Components: Pentium 4
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Northbridge:
 Handles memory
 Graphics
Southbridge:
 PCI bus
 Disk controllers
 USB controllers
 Audio
 Serial I/O
 Interrupt controller
 Timers
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Sun Enterprise 6000 Device-Transfer Rates
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How does processor talk to device?
Memory / I/O Bus
CPU
Interrupt
Controller
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Bus
Adaptor
Other Devices
or Buses
Regular
Memory
Bus
Adaptor
Address+
Data
Interrupt Request
Device
Controller
Bus
Interface
read
write
control
status
Registers
(port 0x20)
Hardware
Controller
Addressable
Memory
and/or
Queues
CPU interacts with a Controller
 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: explicit in/out instructions
 Example from the Intel architecture: out 0x21,AB
 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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Device I/O Port Locations on PCs (partial)
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Memory-Mapped I/O (1)
(a)
(b)
(c)
Separate I/O and memory space.
Memory-mapped I/O.
Hybrid: control in I/O address, data in memory
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Memory-Mapped I/O (2)
(a) A single-bus architecture.
(b) A dual-bus memory architecture.
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Memory-Mapped I/O vs. Explicit I/O Instructions
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Explicit I/O Instructions:
 Must use assembly language
 Prevents user-mode I/O
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Memory-Mapped I/O:
 No need for special instructions (can use in C)
 Allows user-based I/O
 Caching addresses must be prevented
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Notifying the OS: Polling
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The OS needs to know when:
 The I/O device has completed an operation
 The I/O operation has encountered an error
One way is to Poll:
 OS periodically checks a device-specific status
register
 I/O device puts completion information in status
register
 Busy-wait cycle to wait for I/O from device
 Pro: simple, potentially low overhead
 Con: may waste many cycles on polling if
infrequent, expensive, or unpredictable I/O
operations
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Notifying the OS: Interrupts
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I/O Interrupt:
 Device generates an interrupt whenever it needs
service
 Pro: handles unpredictable events well
 Con: interrupts relatively high overhead
Some devices combine both polling and interrupts
 Example: Intel E1000 Gigabit Ethernet Adapter:
 Interrupt for first incoming packet
 Poll for subsequent packets until hardware
packet arrival queue is empty
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Interrupts (Revisited)
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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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Precise and Imprecise Interrupts (1)
Properties of a precise interrupt:
1.
2.
3.
4.
PC (Program Counter) is saved in a known
place.
All instructions before the one pointed to by
the PC have fully executed.
No instruction beyond the one pointed to by
the PC has been executed.
Execution state of the instruction pointed to
by the PC is known.
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Precise and Imprecise Interrupts (2)
(a) A precise interrupt. (b) An imprecise interrupt.
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Transfering Data To/From Controller
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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
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Programmed I/O (1)
Steps in printing a string.
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Programmed I/O (2)
Writing a string to the printer using programmed I/O.
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Direct Memory Access (DMA)
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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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Application I/O Interface
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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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Device Drivers
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Device-specific code
in the kernel that
interacts directly with
the device hardware
 Supports a
standard, internal
interface
 Same kernel I/O
system can
interact easily with
different device
drivers
 Special devicespecific
configuration
supported with the
ioctl() system
call
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Device Drivers
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Device Drivers typically divided into two pieces:
 Top half: accessed in call path from system calls
 Implements a set of standard, cross-device calls
like open(), close(), read(), write(),
ioctl()
 This
is the kernel’s interface to the device driver
 Top half will start I/O to device, may put thread to
sleep until finished
 Bottom half: run as interrupt routine
 Gets input or transfers next block of output
 May wake sleeping threads if I/O now complete
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Uniform Interfacing for Device Drivers
(a)
(b)
Without a standard driver interface.
With a standard driver interface.
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Characteristics of I/O Devices
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Block and Character Devices
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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
 Single character at a time
 Commands include get, put
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Libraries layered on top allow line editing
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Network Devices
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Different 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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Clock Devices and Timers
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Maintaining the time of day
Accounting for CPU usage
Preventing processes from running longer than they
are allowed to
Handling alarm system call made by user
processes
Providing watchdog timers for parts of the system
itself.
Doing profiling, monitoring, statistics gathering
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Clock Hardware
A programmable clock.
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Clock Software (2)
Three ways to maintain the time of day.
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Clock Software (3)
Simulating multiple timers with a single clock.
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Blocking and Nonblocking I/O
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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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Two I/O Methods
Asynchronous
Synchronous
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Kernel I/O Subsystem
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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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Buffering (1)
(a) Unbuffered input.
(b) Buffering in user space.
(c) Buffering in the kernel followed by copying to user space.
(d) Double buffering in the kernel.
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Buffering (2)
Networking may involve many copies of a packet.
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UNIX I/O Kernel Structure
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Kernel keeps
state for I/O
components,
including open
file tables,
network
connections,
character
device state
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I/O Requests to Hardware Operations
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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
User
Program
Kernel I/O
Subsystem
Device Driver
Top Half
Device Driver
Bottom Half
Device
Hardware
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Device-status Table
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I/O Subsystem Example: STREAMS
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STREAM – a full-duplex communication channel
between a user-level process and a device in Unix
System V
A STREAM consists of:
- STREAM head interfaces with the user process
- Driver end interfaces with the device
- Zero or more STREAM modules between them.
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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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I/O Performance
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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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Improving performance:
 Use DMA
 Reduce/eliminate data copying
 Reduce number of context switches
 Reduce interrupts by using large transfers,
smart controllers, polling
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Interaction of I/O and VM
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The kernel deals with (kernel) virtual addresses
These do not necessarily correspond to physical
addresses
Contiguous virtual addresses are probably not
contiguous in physical memory
Some systems have an I/O map — the I/O bus manager
has a (version of) the virtual memory map
More often, the kernel has to translate the virtual
addresses itself
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Other I/O Issues
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Scatter/Gather I/O
 Suppose we’re reading a single packet or disk block into two or
more non-contiguous pages
 The I/O transfer has to use more than one (address, length) pair
for that transfer to scatter the data around memory
 The same applies on output, where it has to be gathered from
different physical pages

Direct I/O
 For efficiency, we may want to avoid copying data to/from user
space
 Sometimes possible to do direct I/O
 Must consult user virtual memory map for translation
 Must lock pages in physical memory during I/O
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End of Chapter 13
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Kernel I/O Subsystem
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Caching - fast memory holding copy of data
 Always just a copy
 Key to performance
Spooling - hold a copy of 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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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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Device-Functionality Progression
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Error Handling
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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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Device-Independent I/O Software
Functions of the device-independent I/O software.
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User-Space I/O Software
Layers of the I/O system and the main functions of each
layer.
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