Chapter 13: I/O SystemsChapter 13: I/O Systems
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Chapter 13: I/O SystemsChapter 13: I/O Systems
I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations
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ObjectivesObjectives
Explore the structure of an operating system’s I/O subsystem Discuss the principles of I/O hardware and its complexity
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OverviewOverview
The control of devices connected to the computer is a major concern of operating-system designers.
I/O devices vary so widely in their function and speed, varied methods are needed to control them.
Two conflicting trends of I/O-device technology: Increasing standardization of software and hardware interfaces An increasingly broad variety of I/O devices.
To encapsulate the details and oddities of different devices, the kernel of an operating system is structured to use device-driver modules
The device drivers present a uniform device-access interface to the I/O subsystem
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I/O HardwareI/O Hardware Incredible variety of I/O devices Common concepts
Port: a connection point Bus: a set of wires and a rigidly defined protocol that specifies
a set of messages that can be sent on the wires Controller: a collection of electronics that can operate a port, a
bus, or a device. I/O instructions control devices
The controller has one or more registers for data and control signals. The processor communicates with the controller by reading or writing bit patterns in these registers
How can the processor give commands and data to a controller to accomplish an I/O transfer? Direct I/O instructions Memory-mapped I/O
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A Typical PC Bus StructureA Typical PC Bus Structure
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Device I/O Port Locations on PCs (partial)Device I/O Port Locations on PCs (partial)
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PollingPolling Determines state of device
command-ready indicates command is available for the controller to execute
Busy indicates the device is busy working Error indicates there is something wrong
Basic handshaking notion: The host repeatedly reads the busy bit until it is clear The host sets the writes bit in the command register and writes a byte into the
data-out register The host sets the command-ready bit When the controller notices that the command-ready bit is set, it sets the busy bit The controller reads the command register and sees the write command. It reads
the data-out register to get the byte and does the I/O to the device. The controller clears the command-ready bit, clears the error bit in the status
register to indicate that the device I/O succeeded, and clears the busy bit to indicate that it is finished
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Interrupt-Driven I/O CycleInterrupt-Driven I/O Cycle
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InterruptsInterrupts
The hardware mechanism that enables a device to notify the CPU is called an interrupt
Interrupt handler receives interrupts Most CPU has two Interrupt-request line triggered by I/O device:
nonmaskable and maskable Nonmaskable is reserved for event such as unrecoverable
memory erros Maskable to ignore or delay some interrupts
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Application I/O InterfaceApplication 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 StructureA Kernel I/O Structure
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Characteristics of I/O DevicesCharacteristics of I/O Devices
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Block and Character DevicesBlock and Character Devices
Block devices include disk drives Commands include read, write, seek Raw I/O or file-system access for OS and database-
management systems Memory-mapped file access possible
Character devices include keyboards, mice, serial ports Commands include get and put Libraries layered on top allow line editing
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Network DevicesNetwork 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: returns information about which
sockets have a packet waiting to be received and which sockets have room to accept a packet to be sent
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Clocks and TimersClocks and Timers
Provide current time, elapsed time, timer to trigger a certain operation at time T
Programmable interval timer used for timings, periodic interrupts
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Blocking and Nonblocking I/OBlocking 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
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Two I/O MethodsTwo I/O Methods
Synchronous Asynchronous
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Kernel I/O SubsystemKernel I/O Subsystem
Scheduling Some I/O request ordering via per-device queue: a waiting queue of
request for each device Some OSs try fairness: rearrange the order of the queue to improve
the overall system efficiency and the average response time
Buffering - store data in memory while transferring between devices or between a device and an application To cope with device speed mismatch To cope with device transfer size mismatch To maintain “copy semantics”: the version of the data written to disk
is guaranteed to be the version at the time of the application system call, independent of any subsequent changes in the application’s buffer
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Device-status TableDevice-status Table
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Sun Enterprise 6000 Device-Transfer RatesSun Enterprise 6000 Device-Transfer Rates
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Kernel I/O SubsystemKernel 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 HandlingError 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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Kernel Data StructuresKernel 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 StructureUNIX I/O Kernel Structure
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I/O Requests to Hardware OperationsI/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 RequestLife Cycle of An I/O Request
End of Chapter 13End of Chapter 13