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I/O Management

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Differences in I/O Devices

Data transfer rate

Complexity ofcontrol

Unit oftransfer data may be transferred as

a stream ofbytes (e.g. terminal), stream-oriented

in larger blocks (e.g. disk), block-oriented

Data representation Error conditions

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3

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Objectives and implication

Chap 11 4

Blocking - process suspended until I/O completed

Easy to use and understand

Insufficient for some needs

Nonblocking - I/O call returns as much as available

User interface, data copy (buffered I/O)

Implemented via multi-threading

Returns quickly with count of bytes read or written

Asynchronous - process runs while I/O executes

Difficult to use

I/O subsystem signals process when I/O completed

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Performing I/O

Programmed I/O

Process is busy-waiting for the operation tocomplete

Interrupt-driven I/O

I/O command is issued

Processor continues executing instructions

I/O module sends an interrupt when done

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Performing I/O

Direct Memory Access (DMA)

DMA module controls exchange ofdatabetween main memory and the I/O device

Processorinterrupted only after entire block

has been transferred

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DMA Configurations

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DMA Configurations

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Techniques forPerforming I/O

Programmed I/O Interrupt-driven I/OI/O-to-memorytransfer through

processor

DMADirect I/O-to-

memory transfer

No Interrupts Use of Interrupts

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Operating System Design Issues

Efficiency

Most I/O devices extremely slow comparedto main memory

Use ofmultiprogramming allows for some

processes to be waiting on I/O while another

process executes I/O cannot keep up with processor speed

Swapping is used to bring in additional

Ready processes which is an I/O operation

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Operating System Design Issues

Generality

Desirable to handle all I/O devices in auniform manner

Hide most ofthe details ofdevice I/O in

lower-level routines so that processes and

upper levels see devices in general terms suchas read, write, open, close, lock, unlock

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Buffer Buffering - store data in memory while transferring

between devices

To cope with device speed mismatch

To cope with device transfer size mismatch

To maintain copy semantics

Chap 11 12

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I/O Buffering

Reasons for buffering

Processes must wait for I/O to complete before

proceeding

Certain pages must remain in main memory during

I/O

Chap

Addressing space of a process

...fgets(F,s1)fgets(F,s2)...

s1

z

I/O device

Chap 11Input / Output

s2

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How I/O Buffer works, 1

Addressing space of a process

...fgets(F,s1)fgets(F,s2)...

s1

z

I/O device

Chap 11

Input / Output

s2

To avoid these overheads and

inefficiencies, it is sometimes convenient

to perform input transfers in advance of

requests being made and to perform

output transfers sometime after the

request is made. This technique is

known as buffering.

Cha

I/O buffer

OS reads data from I/O

device to I/O buffer. User

process reads from I/O buffer.

(similar for data write)

I/O buffer is not swapped out.

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How I/O Buffer works, 2Chap 11

Addressing space of a process

...fgets(F,s1)fgets(F,s2)...

s1

I/O device

Chap 11

Input / Output

s2

When the process requests the first line, it is

blocked. A few lines will be read, in advance, from

the I/O device to the I/O buffer. Later, when the

process requests the second line, the OS can

satisfy the request from the I/O buffer without

blocking.

Chap 11

Input / Output

Input/Output

I/O buffer

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I/O Buffering

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Double Buffer

Use two system buffers instead ofone

A process can transfer data to orfrom one buffer

while the operating system empties orfills the

other buffer

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Chap 11 19

Spooling

hold output for a device If device can

serve only one request at a time

i.e., Printing