I/O Devices

CSC501 Operating Systems Principles

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Previous Lectures

q AbstractionQ ProcessQ Memory

q NowQ I/O Devices

Basic Computer Architecture

CPU

memory

I/O Devices

CPUSystem Bus &MMU/AGP/PCI

Controller

I/O Bus

IDE DiskController

USBController Another

I/O BusSerial & Parallel Ports

Keyboard & Mouse

Intel SR440BX MotherboardMemory

q The largest, most complex subsystem in OSq Most lines of code with the highest rate of code

changesQ Where OS engineers most likely to workQ Difficult to test thoroughlyQ Big impact on performance and perceptionQ Bigger impact on acceptability in market

OS Designer’s Perspective

Principles of I/O Software

q Efficiency - Do not allow I/O operations to become system bottleneck, especially for slow devices

q Generality - Provide a common API to access I/O devicesQ Uniform naming/interfacev open, close read, write…

Q Device independencev isolate OS and application programs from device

specific details and peculiaritiesv isolate the impact of device errors, retry where

possiblen Errors are abundant in I/O

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I/O Software “Stack”

User Level Software

Device IndependentSoftware

Device Drivers

Interrupt Handlers

Hardware

I/O API & libraries

Device Dependent

Device Dependent – as

short as possible

(Rest of the OS)

Communication with I/O Devices

q Programmed I/O (a.k.a., polling)q Interrupt-Driven I/Oq DMA-based I/O

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Programmed I/O (Polling)

q Used when device and controller are relatively quick to process an I/O operationQ Device driverv Gains access to devicev Initiates I/O operationv Loops testing for completion of I/O operationv If there are more I/O operations, repeat

Q Used in following kinds of cases v Service interrupt time > Device response timev Device has no interrupt capabilityv Embedded systems where CPU has nothing else to do

Any Example?

Interrupt-Driven I/Oq Interrupts occur on I/O events Q Operation completion Q Error or change of status

q Interrupt handlingQ Step 1: Save the current machine stateQ Step 2: Load the machine state for interrupt

handlingQ Step 3: Invoke the corresponding ISRQ Step 4: Resume the program execution

Any Example?

Question: Where do we save the current machine states?

DMA-Based I/O

q Direct Memory AccessQ Device read/write directly from/to memoryQ Transfer from memory to device typically

initiated from CPUQ Transfer from device to memory can be

initiated by the device or the CPU

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Any Example?

DMA-Based I/O

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q Comparison: Polling, Interrupt, and DMAQ Polling and Interrupt = word at a timeQ DMA = block at a time

Communication with I/O Devices

CPU Memory

Disk

Interconnect

CPU Memory

Disk

Interconnect

CPU Memory

Disk

Interconnect

Polling Interrupt DMA

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Question:

How should the device driver be designed?

Device Driver

q Hides the device specifics from the above layersQ Supporting a common API

q Translates logical I/O into device I/OQ E.g., logical disk blocks into {head, track, sector}Q Performs data buffering and scheduling of I/O

operationsq StructureQ Several synchronous entry points: device

initialization, queue I/O requests, state control, read/write

Q An asynchronous entry point to handle interrupts

Device Driver – A Typical Design

q OS and driver communication: OS-dependentQ Commands and data between OS and driversv Meet the interface specs of the device independent

layerv Utilize the facilities supplied by the OS – buffers,

error codes, etc.q Driver and hardware comm.: Device-dependentQ Commands and data between driver and hardwarev Actions during interrupt handling

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Device Driver – A Typical Design

q Driver operationsQ Initialize devicesQ Interpret commands from OSQ Schedule multiple outstanding requestsQ Manage data transferQ Accept and process interruptsQ …

q Typically use open-close-read-write paradigmQ Support open, close, read, write, seek functionsQ Use ioctl function as escape mechanism

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Uniform API (e.g., in Linux)

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Case Study w/ Xinu

q Code divided into two partsQ Upper Halfv Defines abstract operationsv Executed by application to request I/O

Q Lower Halfv Device-specific interrupt routinev Executed by arbitrary (current) processv Invoked when operation completes

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User

system call Device

specific upper-half

routine

Device specific

lower-half routine

Device

Device switch table devtab[ ]

Case Study w/ Xinu

q Device switch tableQ One row per deviceQ One column per operationQ Entry points to a procedure

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int dvnum;

long int (*dvinit)();

long int (*dvshutdown)();

long int (*dvopen)();

long int (*dvclose)();

long int (*dvread)();

long int (*dvwrite)();

long int (*dvseek)();

long int (*dvgetc)();

long int (*dvputc)();

long int (*dvcntl)();

long int (*dviint)();

long int (*dvoint)();

long int dvcsr;

long int dvivec;

long int dvovec;

char *dvioblk;

int dvminor;

device switch table entry ( struct devsw)

Ptrs to device specific high-level

routines

The structure of the control block varies from device to device. The control block contains information associated with the device.

Ptr to device control block

Device Switch Table

Case Study w/ Xinu

q Device switch tableQ Device-independent operations ‘‘generic’’Q Given operation may not apply to given devicev Example: seek on keyboard or network

Q All entries in device switch table must be valid

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Any solution?

Case Study w/ Xinu

q Device switch tableQ Device-independent operations ‘‘generic’’Q Given operation may not apply to given devicev Example: seek on keyboard ornetwork

Q All entries in device switch table must be validq Special EntriesQ ionullv Used for innocuous operation, always returns OK

Q ioerrv Used for incorrect operation, always returns

SYSERR

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Example Device Driver tty in Xinu

q Used for I /O to consoleq Separate input and output buffersq Use semaphores to synchronize

q Upper-half routinesQ ttyinit, ttyopen, ttyclose, ttyread, ttywrite,

ttyputc, ttygetc, ttypcntlq Lower-harf routinesQ ttyiin (input interrupt handler) Q ttyoin (output interrupt handler)

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Example Device Driver tty in Xinu

q Actions during a character outputQ Upper-halfv Waits for buffer spacev Deposits characterv Triggers device

Q Lower-halfv Extracts character and pass to devicev Signals space semaphore

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Next Lecture

q Hard Disk Q Scheduling and RAID

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