Chapter 13: I/O Systems

Chapter 13: I/O SystemsChapter 13: I/O Systems

13.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts – 7th Edition, Jan 2, 2005

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


ObjectivesObjectives

Explore the structure of an operating system’s I/O subsystem Discuss the principles of I/O hardware and its complexity


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


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


A Typical PC Bus StructureA Typical PC Bus Structure


Device I/O Port Locations on PCs (partial)Device I/O Port Locations on PCs (partial)


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


Interrupt-Driven I/O CycleInterrupt-Driven I/O Cycle


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


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


A Kernel I/O StructureA Kernel I/O Structure


Characteristics of I/O DevicesCharacteristics of I/O Devices


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


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


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


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


Two I/O MethodsTwo I/O Methods

Synchronous Asynchronous


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


Device-status TableDevice-status Table


Sun Enterprise 6000 Device-Transfer RatesSun Enterprise 6000 Device-Transfer Rates


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


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


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


UNIX I/O Kernel StructureUNIX I/O Kernel Structure


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


Life Cycle of An I/O RequestLife Cycle of An I/O Request

End of Chapter 13End of Chapter 13

Chapter 13: I/O Systems

Documents

io transfer

io subsystem13

io systems13

principles of io hardware

device io port locations

conflicting trends of

broad variety of io

interruptdriven io cycle13