I/O and Device Drivers

Minsoo Ryu

Real-Time Computing and Communications Lab.

Hanyang University

msryu@hanyang.ac.kr

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Topics Covered

I/O Components

I/O Interface

I/O Operations

Device Drivers

I/O Components

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CPU Execution and I/O

The two main jobs of a computer

▪ CPU execution

▪ Input/Output

Usually, the main job is I/O

▪ The CPU execution is incidental

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Typical I/O Data Rates

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Variety of I/O Devices

A computer is equipped with

▪ CPU and memory

▪ Many I/O devices

• VGA card, network card, disk controller, …

Main board

CPU

Memory

Network card

Hard disk

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Types of I/O Devices

Character devices (mouse, terminal, etc)

▪ Commands include get and put

Block devices (disk drive, flash drive, etc)

▪ Commands include read, write, and seek

▪ Raw I/O or file-system access

▪ Memory-mapped file I/O access possible

• mmap( ) function: convenient programming interface

• mmap is faster than read, because of single copy operation

Network devices

▪ NIC (network interface card)

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Character Device: Mouse (1)

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Character Device: Mouse (2)

Brief history

▪ First mechanical mouse with a roller ball

• Bill English at Xerox PARC in the early 1970s

▪ Introduced by Apple Macintosh in 1984

• They have helped to completely redefine the way we use computers

since then

▪ Became the PC-human interface of choice quickly when

Windows 3.1 made Graphical User Interface (GUI) a standard

▪ Optical Mouse

• Gary Gordon at Agilent Laboratories in 1999

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Character Device: Mouse (3)

Optical mouse

▪ Tiny camera takes 1500-7080 images per second

• Camera = laser + a CMOS sensor

▪ Images sent for analysis to a DSP operating typically at 18

MIPS

▪ DSP detects patterns in images and thus estimates motion

▪ Data ports are used for two-way communication

▪ Upon mouse movement, a 3/5-byte packet is sent to the port

• Typical description of the data

✓ (xs, ys), (xd, yd), mouse-up/down

▪ This data packet is decoded by the mouse driver and its

internal co-ordinates are updated

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Character Device: Mouse (4)

PC mouse system (data transfer chain)

▪ Sensors (CMOS)

▪ Mouse Controller (DSP)

▪ Communication link (Cable/Wireless)

▪ Data interface (Serial, PS/2, USB)

▪ Device driver

▪ Application

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Character Device: Mouse (5)

How OS collaborate with mouse driver

▪ Applications wait for mouse movement or click

▪ When a mouse movement occurs, the mouse driver informs

the event manager of OS about the event

• Event manager determines whether to queue the event or not

• Mouse driver automatically tracks the mouse and displays the cursor

as the user moves the mouse

▪ When a mouse-up or mouse-down event occurs, the event

manager records the action in the event queue and informs

the active application about it

▪ The active program decides what action is to be taken

• Ex: Show the mouse cursor, hide the cursor and draw something onto

screen, etc.

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Character Device: Terminal (1)

Example

▪ DEC VT100, Heathkit Z19

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Character Device: Terminal (2)

Terminal = keyboard + display

▪ Keyboard and display are handled independent in most

systems (no automatic echo, full duplex serial link)

I/O registers are connected to host via serial line

▪ Keyboard data/status registers

▪ Display data/status registers

One interrupt per character

▪ One character (8-bit data or control function) is sent at a time

ASCII encoding is used: ‘A’ is 0x65

Slow speed

▪ 10-1800 characters per second

▪ Measure: Baud rate (bits per second)

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Character Device: Terminal (3)

Keyboard input

▪ When a character is typed:

• Its ASCII code is placed in bits [7:0] of keyboard data register

• The “ready bit” of keyboard status register is set to zero

• Keyboard is disabled

✓ Any typed characters will be ignored

▪ When KBDR is read:

• The “ready bit” of KBSR is set to one

• Keyboard is enabled

KBSR

KBDR15 8 7 0

1514 0

keyboard data

ready bit

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Character Device: Terminal (4)

Display output

▪ When monitor is ready to display another character

• The “ready bit” of display status register is set to one

▪ When data is written to DDR:

• The “ready bit” of DSR is set to zero

• Character in DDR is displayed

• Any other character data written to DDR is ignored

DSR

DDR15 8 7 0

1514 0

display data

ready bit

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Character Device: Terminal (5)

Keyboard echo

▪ Usually, input character is also printed to screen

▪ User gets feedback on character typed and knows it’s ok to

type the next character

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Block Device: Disk Drive (1)

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Block Device: Disk Drive (2)

Disk characteristics (technology of 2012)

▪ 2-6 heads (platters x 2)

▪ Platter diameter between 0.8" and 8"

▪ 16,383 tracks (cylinders) per surface

▪ 63 sectors per track

▪ Sector size of 512 to 4096 bytes

• 4KB physical emulated at 512-byte sectors

▪ Capacity ranges up to 4 TB

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Block Device: Disk Drive (3)

Disk operation

▪ Select desired read/write head

▪ Move heads to the correct track (“seek”)

• Seek time

▪ Wait for disk to rotate desired sector into position

• Rotational delay

▪ Read and write sector while it spins by

• Transfer latency

Disk performance

▪ Seek time: 0-50 ms (average 10-20 ms)

▪ Rotational delay: 0-16 ms

▪ Typical drive spins at 3600-5400 RPM

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Network Interfaces

Network interfaces

▪ UART and RS-232, USB, ethernet, etc

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Layering (Protocol Stack)

Data

DataTCP

DataTCPIP

DataTCPIPEthernet

Network

Adapter (NIC)

Message

Segment/Datagram

Datagram

Frame

Bit stream

Port no.

IP address

MAC address

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TCP Socket Programming Flow

ServerClient socket()

listen()

accept()

read()

bind()

Block until

connection from client

Process requests

write()

read()

close()

socket()

write()

connect()

read()

close()

well-known

port

Connection establishmentTCP three-way handshake

Data (request)

Data (reply)

End-of-file notification

Implicit bind

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Clocks and Timers

Provide three basic functions▪ Give the current time

▪ Give the elapsed time

▪ Set a timer to trigger operation X at time Y

Programmable interval timer▪ The hardware for last two functions

▪ Wait and generate an interrupt

▪ CPU scheduler uses this

Unfortunately, the system calls for timer functions are not standardized across operating systems▪ alarm() in UNIX

▪ timer_create() and timer_settime() in POSIX

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Unix Time Values

Two different time values

▪ Calendar time

• Counts the number of seconds since the Epoch (Jan 1, 1970)

• UTC (coordinated Universal Time, Greenwich Mean Time)

▪ Process time

• Measures the CPU time used by a process

• Process time is measured in clock ticks

✓ Usually, 50, 60, or 100 ticks per second

• CPU time = User CPU time + System CPU time

Lack of timer support on Unix

▪ We can get around by using alarm( ) system call

• The unit is second

▪ ioctl on UNIX covers odd aspects of I/O such as clocks and timers

• Catchall for I/O operations

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Linux

The timer interrupt is set to a default frequency

▪ “linux/param.h”

▪ 100 Hz for most hardware platforms

Jiffies

▪ The number of clock ticks since the operating system was

booted

▪ When the timer interrupt occurs, the jiffies value is

incremented

I/O Interface and Operations

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I/O Bus and Connection

Devices are attached to I/O bus

▪ ISA, PCI, EISA, SCSI, …

I/O BUS

CPU VGA Ethernet Sound

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I/O Buses and Connections

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Generic Model of I/O Interface

I/O Interface

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I/O Registers

An I/O port typically consists of four registers

▪ Status register: states read by the host

• Whether the current command has completed

• Whether a byte is available to be read from data-in register

• Whether there is a device error

▪ Control register: command written by the host

• Start a command and change the mode of a device

▪ Data-in register: input read by the host

▪ Data-out register: output written by the host

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I/O Register Access

Memory-mapped IO▪ Data Bus, Address Bus, Control Bus are shared

▪ e.g., Motorolla 68000

▪ Memory and I/O access use the same instructions

Port-mapped IO▪ Data Bus and Address Bus are shared, but Control Bus not

shared

▪ e.g., Intel x86 and Z80

▪ Memory and I/O access use different instructions

Data Channel▪ Separate Data Bus, Address Bus, and Control Bus

▪ Dedicated I/O processors are used

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Memory-Mapped I/O

Memory mapped I/O uses special memory locations in the normal address space

Possible to design address decoder to “divert” read or write to a device instead of memory

Use move instead of in and out

PCs use both port-mapped mapped I/O and memory-mapped I/O

▪ Graphics controller has I/O ports for basic control operations

▪ But it also has a large memory-mapped region to hold screen contents

▪ The controller generates the screen image based on the contents of this memory

▪ Memory-mapped I/O is vulnerable to accidental modification by using pointers, but protected memory reduces this risk

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Port-mapped I/O and Address Space

Port-mapped IO structure provides two address

spaces

▪ Memory address space and I/O (or port) address space

▪ The I/O address space uses the same address pins on the

processor as the memory address space

❖ RD and WR lines determine

whether data being read or written

❖ When M/IO line is high,

memory is selected

❖ When M/IO line is low,

I/O is selected

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Instructions for Port-Mapped I/O

Port-mapped input/output uses special instructions

Register I/O instructions

▪ e.g., in and out (80386)

▪ Move data between I/O ports and general registers

▪ Data may be a byte, word, or doubleword

Block I/O instructions

▪ e.g., ins and outs (80386)

▪ Move blocks of data between I/O ports and memory space

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Device I/O Port Locations on PCs (partial)

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Basic I/O Operations

1. CPU checks I/O module status

2. I/O module return status

3. If ready, CPU requests data transfer

4. I/O module gets data from device

5. I/O module transfers data to CPU

❖ Variations for output such as DMA

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Two Types of I/O

Polling (Programmed I/O)

▪ The system repeatedly checks the status of the I/O module if

data can be read or written

▪ Usually used for character devices

Interrupts (Interrupt-Driven I/O)

▪ The system sleeps the process until data can be read or

written and wakes it up upon receiving the awaited interrupt

▪ Usually used for character and block devices

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DMA (Direct Memory Access)

Interrupt driven and programmed I/O require active

CPU intervention

▪ Transfer rate is limited

▪ CPU is tied up

DMA (Direct Memory Access)

▪ DMA controller is required

▪ Bypasses CPU to transfer data directly between I/O device

and memory

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DMA Controllers

To do DMA, I/O device attached to DMA controller

▪ Multiple devices can be connected to one controller

▪ Controller itself seen as a memory mapped I/O device

• Processor initializes start memory address, transfer size, etc.

▪ DMA controller takes care of bus arbitration and transfer

details

• That’s why buses support arbitration and multiple masters

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Cycle Stealing

When the DMA controller seizes the memory bus,

▪ The CPU is momentarily prevented from accessing main

memory

▪ But, it can still access data items in its primary and

secondary cache

The cycle stealing can slow down the CPU

computation

However, offloading the data-transfer work to a DMA

controller generally improves the total system

performance

Device Drivers

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Types of Linux Devices

Character Devices

▪ Transfer unit: character (byte)

▪ Can be accessed like a file (open, close, read, write, …)

▪ (Ex) console, keyboard, mouse, … (/dev/tty1, /dev/lp1, …)

Block Devices

▪ Transfer unit: block (usually some kilobytes)

▪ Can be accessed like a file (open, close, read, write, …)

▪ (Ex) hard disk drive, CD-ROM drive, … (/dev/hda1, …)

Network Interfaces

▪ Not mapped to neither character or block devices

▪ (Ex) eth0

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Device Files

Represent an I/O device

▪ /dev/tty0 = first serial port

Attributes

▪ Type: block or character

▪ Major number: specifies device driver

▪ Minor number: argument to device driver, kernel don’t care

Name Type Major Minor Desc

/dev/fd0 Block 2 0 Floppy Disk

/dev/had Block 3 0 First IDE disk

/dev/hda2 Block 3 2 Second partition of first IDE disk

/dev/hdb Block 3 64 Second IDE disk

/dev/console Char 5 1 Console

/dev/null char 1 3 Null device

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Structure of Character Device Drivers

Name

File operations

…

Character Device Table

0

1

2

Character Device Driver

Open

Read

Write

Close

Ioctl

…

Read code

write code

Open code

…

Device Name

Registered when

device being installed

Character DeviceIndex =

major number

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Character Device Drivers

To applications

▪ Device file = Regular file

To file system

▪ Regular file: read from or write to disk drive

▪ Device file: invoke device driver operations

• open: initialize device

• read: device data to user buffer

• write: user buffer to device

• close: called when removed from device table

• ioctl: device specific control

✓ (Ex) baud rate change, access permission set

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Structure of Block Device Drivers

Block Device Table Block Device Driver

Open

Read

Write

Close

Ioctl

…

Open code

…Block Device

Handling request code

(strategy routine)

Buffer Cache Manager

Block Read

Block Write

BufferBuffer

BufferBlock Buffer

(Buffer Cache)

Request queue

char

block

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Interrupt Handling in Device Drivers

Register “interrupt handling routine”

▪ Kernel invokes registered handler when interrupt occurs

Linux interrupt handling (two-level processing)

▪ Interrupt handler (top half)

• High priority function (such as acknowledging to PIC)

• Run with interrupt disabled

• Invoked every time interrupt occurred

• Marks a bottom half as active

▪ Bottom half

• Low priority function (such as transferring data from device)

• Run with interrupt enabled

• Execution may be deferred

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Bottom Half

Proc

Int_handler_1

Ack Intr_1, activate bottom_half_1

Bottom_half_1

Intr_2

Int_handler_2

Ack Intr_2, activate bottom_half_1

Proc

Interrupt

enabled

Interrupt

disabled

In this example,

the bottom half is activated twice but executed once

Intr_1

Interrupt

disabled

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General Device Driver Routines (1)

Open

▪ Reads minor number

▪ Initializes the appropriate device

▪ Initializes the device driver internal data structures

▪ If needed, changes the file operation table

(according to the minor number)

▪ Increases the usage counter

▪ If needed, registers the interrupt handler and enable IRQ

Close

▪ Frees dynamically allocated data structures

▪ Decreases the usage counter

▪ Un-registers the interrupt handler

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General Device Driver Routines (2)

Read/Write

▪ Polling vs. interrupt driven

• Polling

✓Checks the device if data can be read or written

✓Usually used for character devices

• Interrupt Driven

✓ Sleeps the process until data can be read or written

✓Usually used for character and block devices

▪ Blocking vs. non-blocking

• Blocking

✓ If data cannot be read or written, waits until ready

• Non-blocking

✓ If data cannot be read or written, immediately returns

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General Device Driver Routines (3)

Character Device: Polling and Blocking I/O

Check Device

Device ready?

No

Read Data

(device user)

Return to User

Yes

Read

Character Device: Non-Blocking I/O

Check Device

Device ready?No

Read Data

(device user)

Return to User

Yes

Read

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General Device Driver Routines (4)

Character Device: Interrupt Driven and Blocking I/O

Check Device

Device Ready?

Sleep process

No

Read

Yes

Read ready interrupt

Wakeup process

Read Data

(device user)

Return to User

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General Device Driver Routines (5)

Block Device: Interrupt Driven and Blocking I/O

Check write buffer

Buffer full ?

Issue Write Command

Sleep process

No

Write

Yes

Sleep process

Wakeup by another

process

Write complete interrupt

Wakeup process

Return to User

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