OSes: 12. IOSys 1 Operating Systems v Objectives –discuss how an OS manages and controls I/O operations and I/O devices Certificate Program in Software.

OSes: 12. IOSys 1

Operating SystemsOperating Systems

ObjectivesObjectives– discuss how an OS manages and controls discuss how an OS manages and controls

I/O operations and I/O devicesI/O operations and I/O devices

Certificate Program in Software DevelopmentCSE-TC and CSIM, AITSeptember--November, 2003

12. I/O Systems(Ch. 12, S&G)

OSes: 12. IOSys 2

ContentsContents

1.1. IssuesIssues

2.2. I/O HardwareI/O Hardware

3.3. Application I/O InterfacesApplication I/O Interfaces

4.4. Kernel I/O SubsystemsKernel I/O Subsystems

5.5. From I/O Requests to HardwareFrom I/O Requests to Hardware

6.6. PerformancePerformance

OSes: 12. IOSys 3

1. Issues1. Issues

I/O is the main job in most computersI/O is the main job in most computers– processing is secondaryprocessing is secondary

An OS must deal with a wide variety of I/O An OS must deal with a wide variety of I/O devices with different properties:devices with different properties:– mouse, hard disk, CD-ROM,mouse, hard disk, CD-ROM,

joystick, keyboard, etc.joystick, keyboard, etc.

continued

OSes: 12. IOSys 4

Increase the standardization of the I/O Increase the standardization of the I/O software and hardware interfaces.software and hardware interfaces.

Support many types of devices.Support many types of devices.

Performance.Performance.

One solutionOne solution::– device driver modules with standard interfacesdevice driver modules with standard interfaces

Principal Design AimsPrincipal Design Aims

OSes: 12. IOSys 5

2. I/O Hardware2. I/O Hardware Some terminology:Some terminology:

– portport: a device’s connection point to the main : a device’s connection point to the main processor (computer)processor (computer)

– busbus: a connection line allowing several devices : a connection line allowing several devices to access the processorto access the processor

– controllercontroller: a chip or circuit board in the device : a chip or circuit board in the device that manages interaction between the device that manages interaction between the device and processorand processor

OSes: 12. IOSys 6

2.1. Typical PC Bus Structure2.1. Typical PC Bus StructureFig. 12.1, p.399

diskdisk

disk

disk

monitor

graphicscontroller SCSI controller

processor

bridge/memorycontroller

cache

memory

disk disk

IDE disk controller

disk disk

expansion bus interface keyboard

parallel port serial port

PCI Bus

Expansion Bus

SC

SI

bus

OSes: 12. IOSys 7

2.2. Controller Features2.2. Controller Features Runs microcode for specific tasksRuns microcode for specific tasks

– e.g. a hard disk controller may do bad-sector e.g. a hard disk controller may do bad-sector avoidance, bufferingavoidance, buffering

Use registers for holding data, control signalsUse registers for holding data, control signals– the processor communicates with the controller by the processor communicates with the controller by

reading/writing to these registersreading/writing to these registers

Registers may be visible to a processor as I/O Registers may be visible to a processor as I/O port addresses.port addresses.

OSes: 12. IOSys 8

2.3. Some PC I/O Port Addresses2.3. Some PC I/O Port Addresses

I/O Address Range (Hex)I/O Address Range (Hex) DeviceDevice000-00F000-00F DMA controllerDMA controller040-043040-043 timertimer320-32F320-32F hard disk controllerhard disk controller378-37F378-37F parallel portparallel port3D0-3DF3D0-3DF graphics controllergraphics controller3F0-3F73F0-3F7 floppy drive controllerfloppy drive controller : : ::

Fig. 12.2, p.400

OSes: 12. IOSys 9

2.4. Registers <--> I/O Port2.4. Registers <--> I/O Port

Typically, an I/O port address is made up Typically, an I/O port address is made up of 4 registers.of 4 registers.

One register is usually 1- 4 bytesOne register is usually 1- 4 bytes

Different bits/bytes of a register are used Different bits/bytes of a register are used for manipulating different parts of the for manipulating different parts of the device.device.

OSes: 12. IOSys 10

statusstatus: read by a processor (host) to see if a : read by a processor (host) to see if a device is ready to execute a command, or has device is ready to execute a command, or has finished a command, etc.finished a command, etc.

control/commandcontrol/command: written by a processor (host) : written by a processor (host) to tell the device to start/change its settings, etc.to tell the device to start/change its settings, etc.

data-indata-in: read by a procesor (host): read by a procesor (host)

data-outdata-out: written by a processor (host): written by a processor (host)

Register NamesRegister Names

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2.5. Handshaking2.5. Handshaking

HandshakingHandshaking is one possible protocol is one possible protocol between a host (processor) and a controller.between a host (processor) and a controller.

ExampleExample: the host writes a byte to the : the host writes a byte to the device:device:– 1. The host repeatedly reads the busy bit of the 1. The host repeatedly reads the busy bit of the

device’s device’s statusstatus register until is it clear/false register until is it clear/false busy-waiting (polling)busy-waiting (polling)

continued

OSes: 12. IOSys 12

– 2. The host sets the write bit of the device’s 2. The host sets the write bit of the device’s commandcommand register and writes a byte into the register and writes a byte into the data-outdata-out register. register.

– 3. The host sets the ready bit of the 3. The host sets the ready bit of the commandcommand register.register.

– 4. When the device’s controller notices the 4. When the device’s controller notices the ready bit, it sets the busy bit of its ready bit, it sets the busy bit of its statusstatus register. register.

continued

OSes: 12. IOSys 13

– 5. The controller sees the write bit of its 5. The controller sees the write bit of its commandcommand register. It then reads the byte from register. It then reads the byte from the the data-outdata-out register, and does the I/O. register, and does the I/O.

– 6. The controller clears the 6. The controller clears the commandcommand ready ready bit, and clears the bit, and clears the statusstatus busy bit. busy bit.

OSes: 12. IOSys 14

Polling a DevicePolling a Device

Polling often requires only 3 CPU Polling often requires only 3 CPU instructions:instructions:– readread the status register the status register

– use a use a logical-andlogical-and operation to extract the operation to extract the busy bitbusy bit

– branchbranch (goto) is not zero (goto) is not zero

continued

OSes: 12. IOSys 15

Polling becomes inefficient if done too Polling becomes inefficient if done too often.often.

An alternative is to have the device An alternative is to have the device interrupt the host (CPU) when it is free.interrupt the host (CPU) when it is free.

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2.6. Interrupt Driven I/O Cycle2.6. Interrupt Driven I/O CycleFig. 12.3, p.403

device driverinitiates I/O

CPU checks for interruptsbetween instructions

CPU receives interrupt,transfers control tointerrupt handler

interrupt handler: processes data, returns

CPU resumesinterruped task

CPU

initiates I/O

input ready, or outputcomplete, or error

generate interrupt signal

I/O Controller12

3

4

56

7

OSes: 12. IOSys 17

More SophisticationMore Sophistication

Defer interrupt handling during critical Defer interrupt handling during critical processing.processing.

Use more efficient ways of finding the Use more efficient ways of finding the right interrupt handler.right interrupt handler.

Multi-level interrupts.Multi-level interrupts.

Multiple interrupt request lines:Multiple interrupt request lines:– nonmaskablenonmaskable and and maskablemaskable lines lines

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Interrupt Vector TableInterrupt Vector Table

An An interrupt vector tableinterrupt vector table is used to map an is used to map an interrupt to a routineinterrupt to a routine– done by treating the interrupt number as an offset in done by treating the interrupt number as an offset in

the tablethe table– called an called an interrupt vectorinterrupt vector

Each entry in the table may be a list (chain) of Each entry in the table may be a list (chain) of handlershandlers– allows overloading of the interrupt numberallows overloading of the interrupt number

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Part of a Pentium Vector TablePart of a Pentium Vector Table

Vector NumberVector Number DescriptionDescription00 Divide errorDivide error33 BreakpointBreakpoint55 Bound range errorBound range error77 Device not availableDevice not available1212 Stack faultStack fault1313 General protectionGeneral protection1414 Page faultPage fault19-3119-31 Intel reservedIntel reserved32-25532-255 Maskable interruptsMaskable interrupts

Fig. 12.4, p.404

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Threads and InterruptsThreads and Interrupts

A threaded kernel is useful for A threaded kernel is useful for implementing multiple interrupt priorities.implementing multiple interrupt priorities.

The Solaris thread scheduler allows low The Solaris thread scheduler allows low priority interrupt handlers (threads) to be priority interrupt handlers (threads) to be pre-empted by high priority interrupt pre-empted by high priority interrupt handlers (threads).handlers (threads).

OSes: 12. IOSys 21

2.7. Direct Memory Access (DMA)2.7. Direct Memory Access (DMA)

When the host’s CPU does device When the host’s CPU does device handshaking, it is called handshaking, it is called Programmed I/OProgrammed I/O (PIO)(PIO)– a waste of CPU resourcesa waste of CPU resources

Instead, offload the work to a DMA controllerInstead, offload the work to a DMA controller– the CPU sends the I/O command to the DMA, the CPU sends the I/O command to the DMA,

then goes on with other workthen goes on with other work

continued

OSes: 12. IOSys 22

Higher CPU performance since:Higher CPU performance since:– less kernel communicationless kernel communication– less context switchingless context switching

AdvantagesAdvantages

OSes: 12. IOSys 23

DMA DiagramDMA Diagram Fig. 12.5, p.407

disk disk disk disk

CPU

cache

CPU/memory busDMA controller memory buffer

IDE disk controller

PCI Bus

OSes: 12. IOSys 24

Cycle StealingCycle Stealing

Cycle stealingCycle stealing occurs when the DMA occurs when the DMA controller uses the CPU memory buscontroller uses the CPU memory bus– the CPU is prevented from accessing memorythe CPU is prevented from accessing memory

– this can slow down the CPU’s computationthis can slow down the CPU’s computation

OSes: 12. IOSys 25

Direct Virtual Memory Access (DVMA)Direct Virtual Memory Access (DVMA)

In DVMA, data is transferred between two In DVMA, data is transferred between two memory-mapped devices (i.e. from memory-mapped devices (i.e. from controller to controller) without requiring controller to controller) without requiring main memorymain memory– no cycle stealing requiredno cycle stealing required

OSes: 12. IOSys 26

3. Application I/O Interfaces3. Application I/O Interfaces

OSes treat I/O devices in a standard way OSes treat I/O devices in a standard way using:using:– abstraction, encapulation, software layeringabstraction, encapulation, software layering

The user/programmer sees a standardized The user/programmer sees a standardized set of I/O functions.set of I/O functions.

Differences are hidden inside kernel Differences are hidden inside kernel modules called modules called device driversdevice drivers..

OSes: 12. IOSys 27

A Kernel I/O StructureA Kernel I/O StructureFig. 12.6, p.409

SCSIdevicedriver

keyboarddevicedriver

mousedevicedriver

PCI busdevicedriver

floppydevicedriver

ATAPIdevicedriver

SCSIdevice

controller

keyboarddevice

controller

mousedevice

controller

PCI busdevice

controller

floppydevice

controller

ATAPIdevice

controller

SCSIdevices

keyboard mouse PCI busfloppy

disk drivesdisk &

tape drives

kernel I/O subsystem

kernel

OSes: 12. IOSys 28

AdvantagesAdvantages

Simplifies OS developer’s work.Simplifies OS developer’s work. Benefits hardware manufacturersBenefits hardware manufacturers

But…But…– different OSes have different ‘standard’ different OSes have different ‘standard’

device driver interfaces.device driver interfaces.

OSes: 12. IOSys 29

3.1. Characteristics of I/O Devices3.1. Characteristics of I/O Devices

AspectAspect VariationVariation ExampleExampledisk-transfer modedisk-transfer mode character character terminalterminal

block block diskdisk

access methodaccess method sequentialsequential modemmodemrandomrandom CD-ROMCD-ROM

transfer scheduletransfer schedule synchronoussynchronous tapetapeasynchronousasynchronous keyboardkeyboard

sharingsharing dedicateddedicated tapetapesharablesharable keyboardkeyboard

: :

Fig. 12.7, p.410

continued

OSes: 12. IOSys 30

AspectAspect VariationVariation ExampleExampledevice speeddevice speed latencylatency

seek timeseek timetransfer ratetransfer ratedelay between opsdelay between ops

I/O directionI/O direction read-onlyread-only CD-ROMCD-ROMwrite-onlywrite-only graphics graphics

ctrllerctrllerread-writeread-write diskdisk

OSes: 12. IOSys 31

3.2. Character and Block Devices3.2. Character and Block Devices

Character device:Character device:– transfer byte by bytetransfer byte by byte

Block device:Block device:– treat blocks of bytes as the transfer unittreat blocks of bytes as the transfer unit

Standard file-system interface:Standard file-system interface:– read(), write(), seek(), etcread(), write(), seek(), etc

Memory mapping:Memory mapping:– useful for executing kernel modules/servicesuseful for executing kernel modules/services

OSes: 12. IOSys 32

3.3. Network Devices3.3. Network Devices

Distinguished due to differences in Distinguished due to differences in addressing and performance (reliablility, addressing and performance (reliablility, data sequencing, speed).data sequencing, speed).

Standard interface:Standard interface:– sockets, message packets, streamssockets, message packets, streams– select() for monitoring collections of socketsselect() for monitoring collections of sockets

continued

OSes: 12. IOSys 33

Other UNIX (inter-process) communication Other UNIX (inter-process) communication features:features:– half-duplex pipeshalf-duplex pipes– full-duplex FIFOs (queues)full-duplex FIFOs (queues)– message queuesmessage queues

OSes: 12. IOSys 34

3.4. Clocks and Timers3.4. Clocks and Timers

Basic uses:Basic uses:– get the current time/the elapsed timeget the current time/the elapsed time– set a timer to trigger an operation at time Tset a timer to trigger an operation at time T

(e.g. time-slice a process)(e.g. time-slice a process)

Timers have coarse resolution (20-60 Timers have coarse resolution (20-60 ticks/sec), which limits the precision of ticks/sec), which limits the precision of triggerstriggers– a CPU can execute ~100M instructions/seca CPU can execute ~100M instructions/sec

OSes: 12. IOSys 35

3.5. Blocking & Nonblocking I/O3.5. Blocking & Nonblocking I/O

Blocking (synchronous) I/OBlocking (synchronous) I/O– the application is suspended until the I/O is the application is suspended until the I/O is

completedcompleted– easy to understand/implementeasy to understand/implement

Nonblocking (asynchronous) I/ONonblocking (asynchronous) I/O– the application does not waitthe application does not wait

continued

OSes: 12. IOSys 36

Some software requires nonblocking I/OSome software requires nonblocking I/O– most games software has to be able to receive most games software has to be able to receive

user input from the keyboard at any time while user input from the keyboard at any time while updating the screen continuouslyupdating the screen continuously

OSes: 12. IOSys 37

Implementing Nonblocking I/OImplementing Nonblocking I/O

ThreadsThreads– place the blocking I/O in one thread, and let the place the blocking I/O in one thread, and let the

other behaviour execute in other threadsother behaviour execute in other threads

Use nonblocking I/O system callsUse nonblocking I/O system calls– they return immediately with whatever was they return immediately with whatever was

availableavailable– e.g. select() called with a time-out of 0e.g. select() called with a time-out of 0

continued

OSes: 12. IOSys 38

Use asynchronous I/O system callsUse asynchronous I/O system calls– they return immediately, but the completed they return immediately, but the completed

I/O result is ‘available’ some time laterI/O result is ‘available’ some time later

– the result can be made ‘available’ in the result can be made ‘available’ in different ways:different ways:

a special variablea special variable raising a signal (software interrupt)raising a signal (software interrupt) calling a callback functioncalling a callback function

(e.g. used in Java’s event model)(e.g. used in Java’s event model)

OSes: 12. IOSys 39

4. Kernel I/O Subsystems4. Kernel I/O Subsystems

These are features built on top of the These are features built on top of the hardware and device-driver infrastructure, hardware and device-driver infrastructure, including:including:– I/O schedulingI/O scheduling– bufferingbuffering– cachingcaching– spoolingspooling

OSes: 12. IOSys 40

4.1. I/O Scheduling4.1. I/O Scheduling

Scheduling (re-ordering) is carried out to:Scheduling (re-ordering) is carried out to:– improve overall system performanceimprove overall system performance

– share device access fairlyshare device access fairly

– reduce average waiting timereduce average waiting time

OSes: 12. IOSys 41

Scheduling Example Scheduling Example

Three jobs want to read from a hard disk. Three jobs want to read from a hard disk. The read arm is currently near the The read arm is currently near the beginning of the disk.beginning of the disk.– job 1: requests a block at the end of the diskjob 1: requests a block at the end of the disk– job 2: requests a block near the startjob 2: requests a block near the start– job 3: requests a block in the middlejob 3: requests a block in the middle

New scheduleNew schedule: jobs 2, 3, 1: jobs 2, 3, 1

OSes: 12. IOSys 42

4.2. Buffering4.2. Buffering

A A bufferbuffer is a memory area that stores data is a memory area that stores data while it is being transferred between two while it is being transferred between two devices, or between a device and a process.devices, or between a device and a process.

OSes: 12. IOSys 43

Three Uses for BufferingThree Uses for Buffering

Cope with speed mismatchesCope with speed mismatches– e.g. between a modem and hard diske.g. between a modem and hard disk– double bufferingdouble buffering may be used may be used

Cope with different data-transfer sizesCope with different data-transfer sizes– e.g. packet transfer over a networke.g. packet transfer over a network

Support Support copy semanticscopy semantics for application I/O for application I/O– e.g. so write() data cannot be changede.g. so write() data cannot be changed

OSes: 12. IOSys 44

Double Buffering IllustratedDouble Buffering IllustratedVUW cs305

Slow producer

Fast consumer

buffer

buffer

harddisk

OSes: 12. IOSys 45

Slow producer

Fast consumer

buffer

buffer

harddisk

OSes: 12. IOSys 46

Slow producer

Fast consumer

buffer

buffer

harddisk

OSes: 12. IOSys 47

4.3. Caching4.3. Caching

A A cachecache is a region of ‘fast memory’ that is a region of ‘fast memory’ that holds copies of data:holds copies of data:– used to speed-up reuse and updatesused to speed-up reuse and updates– e.g. recently read instructions copied from the e.g. recently read instructions copied from the

hard diskhard disk

Cache and buffer difference:Cache and buffer difference:– a buffer may hold the a buffer may hold the onlyonly copy of a data item copy of a data item

OSes: 12. IOSys 48

4.4. Spooling4.4. Spooling

A A spoolspool is a buffer (or buffers) used to hold is a buffer (or buffers) used to hold output for a device while the device is output for a device while the device is processing the current job.processing the current job.

The device can only process one job at onceThe device can only process one job at once– e.g. a printer, tape readere.g. a printer, tape reader

Spool management is often done by a dedicated Spool management is often done by a dedicated system system daemondaemon (process) (process)

OSes: 12. IOSys 49

Spooling DiagramSpooling Diagram VUW, cs305

Program

Program

Program

Printerdriver

spooldaemon

Printerdriver

OSes: 12. IOSys 50

4.5. UNIX I/O Kernel Data Structures4.5. UNIX I/O Kernel Data StructuresFig. 12.9, p.419

file-system record

inode pointerpointers to functions:read(), write(), select(),...

network record

socket d.s. pointerpointers to functions:read(), write(), select(),...

filedescriptor

process open-file

table

user process

system open-file table

activeinode table

network infotable

kernel

OSes: 12. IOSys 51

A variety of I/O components are A variety of I/O components are controlled/monitored through the open controlled/monitored through the open file table.file table.

Details are hidden from the user by Details are hidden from the user by standard abstractions:standard abstractions:– e.g. read() works with files and network e.g. read() works with files and network

socketssockets

OSes: 12. IOSys 52

5. From I/O Requests to Hardware5. From I/O Requests to Hardware

How does the OS convert an I/O request How does the OS convert an I/O request into hardware commands?into hardware commands?

Consider how UNIX reads from a file on Consider how UNIX reads from a file on disk.disk.

OSes: 12. IOSys 53

Lifecycle of an I/O RequestLifecycle of an I/O RequestFig. 12.10, p.422

Request I/O

can satisfyalready?

Send request todevice driver.Block process.

system call

I/O completed

Transfer datato process

userprocess

yes

no

kernelI/O subsystem

return from call

kernelI/O subsystem

continuedtime

OSes: 12. IOSys 54

Carry out request.Send commands tocontroller. Block.

Determine whenI/O completed.

Tell I/O subsystem.

device controller commands

Monitor device.Interrupt when I/O completed

Receive interrupt.Store data in dd buffer.

Signal dd.

I/O completed.Generate interrupt.

devicedriver

interrupthandler

keyboarddevice

controller

time

OSes: 12. IOSys 55

Filename to Disk ControllerFilename to Disk Controller

A UNIX path name has no clear separation A UNIX path name has no clear separation of the device portion:of the device portion:/home/fidji/ad/public_html/index.html/home/fidji/ad/public_html/index.html

UNIX stores a mount table (in UNIX stores a mount table (in /etc/mnttab/etc/mnttab) ) of device name to pathname details:of device name to pathname details:

/dev/dsk/c1t1d0s0 /home/fidji …/dev/dsk/c1t1d0s0 /home/fidji …/dev/dsk/c1t2d0s0 /home/hawai …/dev/dsk/c1t2d0s0 /home/hawai …/dev/dsk/c1t4d0s0 /home/corse …/dev/dsk/c1t4d0s0 /home/corse …

continued

OSes: 12. IOSys 56

The name (e.g. The name (e.g. /dev/dsk/c1t1d0s0/dev/dsk/c1t1d0s0) is ) is looked up in the file-system, and UNIX looked up in the file-system, and UNIX finds:finds:– <major, minor> device numbers<major, minor> device numbers

The The major device numbermajor device number identifies the identifies the device driver type (e.g. character/block device driver type (e.g. character/block transfer).transfer).

continued

OSes: 12. IOSys 57

The The minor device numberminor device number can be used to can be used to index into a device table to find the device index into a device table to find the device controller’s I/O port address or its memory controller’s I/O port address or its memory mapped address.mapped address.

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Stream ModulesStream Modules UNIX System V allows pipelines of driver UNIX System V allows pipelines of driver

code to be assembled dynamically for an code to be assembled dynamically for an application.application.

module 1

module 2

module N

hardware

Applicationstream head

driver end

OSes: 12. IOSys 59

6. Performance6. Performance

I/O is a major factor in system I/O is a major factor in system performance:performance:– it uses the CPU heavily to execute device it uses the CPU heavily to execute device

drivers, schedule processesdrivers, schedule processes– cycle stealingcycle stealing– costs of interrupt processingcosts of interrupt processing– network trafficnetwork traffic

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Network Comms.Network Comms. Fig. 12.11, p.425

char typed

interrupt generated

interrupt handled

device driver

kernel

user process kernel

device driver

network adapter

interrupt generated

interrupt handled

system call completes

contextswitch

state saved

hardware

state saved

context switch

network

contextswitch Sending

System

OSes: 12. IOSys 61

network

packet receive

d

network adapter

interrupt generated

device driver

kernel

networkdaemon

kernel

networksubdaemon

contextswitch

state saved

hardware

network

contextswitch

context switch

ReceivingSystem

OSes: 12. IOSys 62

Ways to Improve I/O EfficiencyWays to Improve I/O Efficiency

Reduce the number of context switches.Reduce the number of context switches.

Reduce the number of data copys to/from Reduce the number of data copys to/from memory.memory.

Reduce the number of interrupts.Reduce the number of interrupts.

continued

OSes: 12. IOSys 63

Offload I/O work to other processors.Offload I/O work to other processors.

Move more processing into hardware.Move more processing into hardware.

Balance the performance of CPU, memory, Balance the performance of CPU, memory, bus, and the I/O.bus, and the I/O.

OSes: 12. IOSys 64

Efficiency ExamplesEfficiency Examples

The telnet daemon in Solaris runs inside a The telnet daemon in Solaris runs inside a kernel thread:kernel thread:– that eliminates context switching between the that eliminates context switching between the

daemon and the kerneldaemon and the kernel

Front-end processorsFront-end processors– e.g. terminal concentratorse.g. terminal concentrators

continued

OSes: 12. IOSys 65

DMA controllersDMA controllers

I/O channelsI/O channels– an I/O channel is a dedicated CPU for an I/O channel is a dedicated CPU for

offloading I/O work from the main CPUoffloading I/O work from the main CPU

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Where to put I/O Functionality?Where to put I/O Functionality?Fig. 12.12, p.426

application code

kernel code

device driver code

device controller code

(hardware)

device code(hardware)

I/O functionalityimplemented using:

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