Ch3- Io Systems

8/7/2019 Ch3- Io Systems

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352 ALL OPERATING SYSTEM 2SECTION # 474 & 475

Prepared by: Fathima Sakeena

CHAPTER 3 I/O SYSTEMS

TEXT BOOK : Operating System Concepts with JAVA, 6th Edition,Silberschatz, Galvin, Gagne (Willey International Edition)


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Chapter 3 I/O Systems

I/O Hardware

Polling

Interrupts

Direct Memory Access

Application of I/O Interface

Block and character devices,

Network devices Clocks and timers

Kernel I/O.

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352 All- Fathima Sakeena


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A typical PC bus structure


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1. PCI (Peripheral component Interconnect)bus connects processor

and memory subsystem with fast devices.

2. Expansion bus connects slow devices such as keyboard, serial and

parallel ports.

3. SCSI ( small computer system Interface)bus connects the disks and

SCSI controller.

How can the processor give commands and data to controller to perform

I/O transfer?

Controller has one or more registers for data and control signals.

Processor communicates with controller by reading writing bit

patterns from these registers

IDE integrated device electronics


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A Typical PC Bus Structure6



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I/O port registers


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I/O port consists four registers status, control, data in, data

out.

1. Status register contains bits that can be read by host. This

bits indicate whether the current command completed, byte is

available in data in register or device error.

2. Control register can be written by host to start new

command or to change the mode of the device.

3. Data in register read by host to get input.

4. Data out register written by host to sent output.

Data registers are typically 1 to 4 bytes, here host refers to any

one of the I/O device.


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Polling


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Communication between host and controller

Controller indicates its status through the busy bit in status register.

Controller sets busy bit when its is working, and clears the busy bit, when it is

ready to accept the next command.

The host sets the command ready bit , when a command in available incommand register.

If the controller is busy , the host is reading the status register over and over

until the busy bit becomes clear. This called as busy waiting or polling.

Polling becomes inefficient , when it is attempted repeatedly, the host is ready

for service but controller is busy. The hard ware mechanism enables the host to notify the CPU, that the device

becomes ready for service.This is called an interrupt.


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Interrupts

The CPU hardware has a wire called the interrupt request line

that CPU senses after executing every instruction.

When CPU detects a interrupt, CPU transfers the control to

interrupt handler routine. Interrupt handler routine performs

the necessary processing, then return to CPU.

Device controller raises an interrupt interrupt signal send via

interrupt request line CPU catches the interrupt

dispatches to interrupt handler handler clears the interrupt

by servicing the device. Two types of interrupts maskable and non maskable.

Non maskable interrupts for unrecoverable memory errors

Maskable for device controllers


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Interrupt-Driven I/O Cycle

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Direct Memory Access

Device wants to transfer large amount of data, it seems wasteful

to use PIO.

PIO programmed IO- CPU watches the status bit and transfers

one byte at a time.

To avoid these problem, the special purpose processor called

Direct memory access (DMA) controller.

DMA transfer is explained in following diagram with six steps.

Handshaking between DMA controller and device controller can

be performed via a pair of lines called DMA- request and DMA acknowledgement.

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Six Step Process to Perform DMA Transfer

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Application 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-streamor block

Sequentialor random-access

Sharable or dedicated

Speedof operation

read-write, read only, or write only

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A Kernel I/O Structure


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Characteristics of I/O Devices

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Block and Character Devices

Block devices include disk drives

Understands the commands - read ( ), write ( ), seek ( )

Applications access a device through file system interface.

Some special applications (like DBMS) access a device as asimple linear block. This is called as raw I/O.

Memory-mapped file access can be layered on top of the

block devices.

Character devices include keyboards, mice, serial ports

Understands the commands - get(), put()

Libraries layered on top allow line editing

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Clocks and Timers

Computers have hardware clocks and timers that provide three

basic functions

Give the current time

Give the elapsed time Set a timer to trigger operation

Programmable interval timer is a hardware used to measure

elapsed time and to trigger operation.

It can be set to wait for certain amount of time then to generate

an interrupt.

It can be set to do this operation once or repeat the process , to

generate periodic interrupts.

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Blocking and Nonblocking I/O

System call interface has two types - blocking I/O or non blocking I/O.

When an application issues a blocking system call,

The execution of the application is suspended

Application is moved from run queue to wait queue.

Complete the I/O operation.

Application is moved back to run queue

This blocking system call is easy to use and understand

When an application issues a non blocking system call,

Does not halt the execution of application

It returns quickly , with a return value indicates how many bytes were

transferred. The non blocking system call is implemented via multi-threading.

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Kernel I/O Subsystem

Kernel provides many services related to I/O - scheduling,

buffering, catching, spooling, device reservation and error

handling.

1. Scheduling

Schedule a set of I/O requests to determine a good order to

execute them.

This is implemented via device queue

2. Buffering - store data in memory while transferring between

two devices

To cope with device speed mismatch

To cope with device transfer size mismatch

To maintain copy semantics

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Kernel I/O Subsystem

3. Caching - fast memory holding copy of data

Always access the copy not the original

4. Spooling - hold output for a device

If device can serve only one request at a time i.e., Printing

5. Device reservation - provides exclusive access to a device

System calls for allocation and de allocation

Watch out for deadlock

6. Error handling

OS can recover from disk read, device unavailable,

transient write failures

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Ch3- Io Systems

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