The part of OS that implements device management is called collectively as device manager, which includes device drivers and device driver infrastructure Device drivers can be complex However, the drivers for different devices

implement a similar interface so that a higher-level component has a uniform interface for different devices

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3

Device InterfaceDevice Interface

…write(…);…

TerminalDriver

TerminalDriver

PrinterDriver

PrinterDriver

DiskDriver

DiskDriver

TerminalController

TerminalController

PrinterController

PrinterController

DiskController

DiskController

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ApplicationProcess

ApplicationProcess

FileManager

FileManager

Device Controller

CommandCommand StatusStatus DataData

Hardware Interface

System Interface

Device-IndependentDevice-Independent

Device-DependentDevice-Dependent

Functions available to application programs

Abstract all devices (and files) to a few interfaces

Make interfaces as similar as possible Block vs character Sequential vs direct access

Device driver implements functions (one entry point per API function)

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6

open Prepare dev for operationclose No longer using the deviceioctl Character dev specific inforead Character dev input opwrite Character dev output opstrategy Block dev input/output opsselect Character dev check for datastop Discontinue a stream output op

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Variable x Register

Data on device

. . .read(dev_I, “%d”, x);y = f(x). . .

Device dev_IMemory CPU

. . .startRead(dev_I, “%d”, x);. . .While(stillReading()) ;y = f(x). . .

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App

I/O Ctlr

t1 t2 t3 t4 t5 t6 t7 t8 t9

9

App 1

App 2

I/O Ctlr

t1 t2 t3 t4

Dashed line is App 1’s activitySolid line is App 2’s activity

Responsible for directly manipulating the hardware devices

Several strategies Direct I/O: CPU stays in control

Use device polling Use interrupts

Memory-mapped I/O Direct memory access

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Direct I/O The CPU is responsible for determining when

the I/O operation has completed and then for transferring the data between the primary memory and the device controller data registers Polling or interrupt-driven I/O

This seems not effective for devices with large transfers, such as a disk drive

Direct Memory Access (DMA)

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Direct memory access controllers Can read/write data directly from/to primary

memory addresses with no CPU intervention after the driver has started the I/O operation

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Direct I/O or DMA for data transferring Polling or interrupt-driven I/O strategies

Direct I/O with polling DMA I/O with polling

Not supported in general Direct I/O with interrupts DMA I/O with interrupts

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read(device, …);

Data

Device Controller

CommandCommand StatusStatus DataData

read function

write function

1

2 3 4

5

Hardware Interface

System Interface

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CommandCommand StatusStatusData 0Data 0

Data 1Data 1

Data n-1Data n-1LogicLogic

busy done Error code . . .. . .busy done 0 0 idle 0 1 finished 1 0 working 1 1 (undefined)

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…// Start the device…While(busy == 1) wait();// Device I/O complete…done = 0;

…while((busy == 0) && (done == 1)) wait();// Do the I/O operationbusy = 1;…

busy done

Sof

twar

eH

ard

war

e

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read(device, …);

Data

Device Controller

CommandCommand StatusStatus DataData

read driver

write driver

1

2

3

4

5Hardware Interface

System InterfaceDevice Status Table

DeviceHandler

DeviceHandler

InterruptHandler

InterruptHandler

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7

8a

8b

9

At the end of each instruction cycle, we check if an interrupt has occurred

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In general, an interrupt-driven system will have a higher CPU utilization than a polling-based system The CPU time on polling by one process may

be utilized by another process to perform computation

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Polling is normally superior from the viewpoint of a single process Because overhead of polling is less

Interrupt-driven I/O approach gives better overall system performance

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I/O-bound process A process’s overall execution time is

dominated by the time to perform I/O operations

Compute-bound process A process’s time on I/O is small compared to

the amount of time spent using the CPU Many processes have I/O-bound and

compute-bound phases

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Compute-bound

I/O-bound

Time

Device drivers Application programming interface

Coordination among application processes, drivers, and device controllers

Performance optimization

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funci(…)

Trap Table

dev_func_i(devID, …) {// Processing common to all devices … switch(devID) { case dev0: dev0_func_i(…);

break; case dev1: dev1_func_i(…);

break; … case devM: devM_func_i(…);

break; };// Processing common to all devices …}

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OtherKernel

services

OtherKernel

services

Entry Points for Device j

open(){…}

read(){…}

etc.

System call interface

Driver for Device j

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int read(…) {// Prepare for I/O save_state(J); out dev#// Done (no return)}

Device driver J

Device ControllerDevice Controller

Interrupt HandlerInterrupt Handler

void dev_handler(…) { get_state(J);//Cleanup after op signal(dev[j]); return_from_sys_call();}

Device interrupt handler J

J

Device status table

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int read(…) { … out dev#// Return after interrupt wait(dev[J}); return_from_sys_call();}

Device driver J

Device ControllerDevice Controller

Interrupt HandlerInterrupt Handler

void dev_handler(…) {//Cleanup after op signal(dev[j]);}

Device interrupt handler J

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Buffering A technique to keep slower I/O devices busy

during times when a process is not requiring I/O operations

Input buffering Output buffering

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Water CompanyCustomer Office

Water Consumers

Water Producer

Delivering Water

Returning the Empties

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ProcessProcess

Controller

Data

Device

ProcessProcess

Controller

B

Device

A

ProcessProcess

Controller

B

Device

A

Unbuffered Process reads bi-1

Controller reads bi

Process reads bi

Controller reads bi+1

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ProcessProcess

Controller

B

Device

A

ProcessProcess

Controller

B

Device

A

BA BA

Har

dwar

eD

rive

r

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- Producer-consumer problem

From data producer

To data consumer

Buf

fer

i

Buf

fer

j

A spool is a buffer that holds output for a device that cannot accept interleaved data streams Printer is an example

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Character devices vs. block devices

Communication devices Sequentially accessed storage devices Randomly accessed storage devices

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GenericController

GenericController

LocalDevice

LocalDevice

CommunicationsController

CommunicationsController

DeviceDevice

Cabling connecting thecontroller to the device

•Printer•Modem•Network

Bus

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SerialDevice

SerialDeviceMemoryMemory

CPUCPU

• Printer• Terminal• Modem• Mouse• etc.

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RS-232 Interface• 9-pin connector• 4-wires• bit transmit/receive• ...

Serial Device (UART)

UART API•parity•bits per byte•etc.

Device Driver• Set UART parms•read/write ops•Interrupt hander

Software on the CPUDevice Driver API

Bus Interface

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NetworkDevice

NetworkDeviceMemoryMemory

CPUCPU

NetworkDevice

NetworkDevice MemoryMemory

CPUCPU

Data NetworkData Network

Logical CommunicationLogical Communication

• Technology focus includes protocols and software (more on this later … Chapter 15 and beyond ...)

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Track (Cylinder)

Sect

or(a) Multi-surface Disk (b) Disk Surface (b) Cylinders

Cylinder (set of tracks)

Access time has three major components Seek time – the time for the disk arm to move

the heads to the cylinder containing the desired sector

Rotational latency – the additional time waiting for the disk to rotate the desired sector to the disk head

Transfer Time - time to copy bits from disk surface to memory

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Several techniques used to try to optimize the time required to service an I/O request

FCFS – first come, first served Requests are serviced in order received No optimization

SSTF – shortest seek time first Find track “closest” to current track Could prevent distant requests from being

serviced

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Scan/Look Starts at track 0, moving towards last track,

servicing requests for tracks as it passes them

Reverses direction when it reaches last track Look is variant; ceases scan in a direction

once last track request in that direction has been handled

Circular Scan/Look Similar to scan/look but always moves in

same direction; doesn’t reverse Relies on existence of special homing

command in device hardware

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Multiprogramming on I/O-bound programs => set of processes waiting for disk

Seek time dominates access time => minimize seek time across the set

Example: Tracks 0:299; Head at track 76, requests for

124, 17, 269, 201, 29, 137, 12

48

49

First-Come-First-Served(76), 124, 17, 269, 201, 29, 137, 12

=> 48 + 107 + 252 + 68 + 172 + 108 + 125 = 880 steps

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Shortest-Seek-Time-First (76), 29, 17, 12, 124, 137, 201, 269

=> 47 + 12 + 5 + 112 + 13 + 64 + 68 = 321 steps

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Scan (76), 124, 137, 201, 269, 299, 29, 17, 12 => 48 + 13 + 64 + 68 + 30 + 270 + 12 + 5

= 510 stepsLook (76), 124, 137, 201, 269, 29, 17, 12 => 48 + 13 + 64 + 68 + 240 + 12 + 5 = 450 steps

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Circular Scan/ Circular Look• Circular Look: (76), 124, 137, 201, 269, 12, 17, 29

48 + 13 + 64 + 68 + home + 12 + 5 + 12 = 222 + home

• Circular Scan: (76), 124, 137, 201, 269, 299, 12, 17, 2948 + 13 + 64 + 68 + 30 + home + 12 + 5 + 12 = 282 + home

While it builds on the same basic principles, it is more complex and with more details There are a lot of device drivers included and

you can also develop your own device drivers for specific purpose devices

Linux divides into char-oriented devices and block oriented devices fs/blk_dev.c fs/char_dev.c

See pp 168-170

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I/O Portion of Native API

I/O M

anag

er

Device Driver

NT

Exec

utiv

e

HAL

Intermediate Driver

File System Driver

Filter Driver

Filter Driver

Data Flow

Device

API model is the same as for a file Extend device management by adding

modules to the stream Device driver is invoked via an

Interrupt Request Packet (IRP) IRP can come from another stream module IRP can come from the OS Driver must respond to minimum set of

IRPs

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