Module2.EBufferedI/O

TimRogers2017

LearningOutcome#2

How?

A. BusTimingAnalysisB. 9S12CMultiplexedBusExpansionC. General-PurposeI/OPortsD. InterruptHandlingE. BufferedI/OF. Buffered,Interrupt-DrivenPrinterDesignExample

“Anabilitytointerfaceamicrocontrollertovariousdevices”

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Objective“BufferedI/O” Why?

DevicesaremuchslowerthanCPUs(yes– eventhe9S12)

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Problems:Dataproduction/consumptionratesaredifferent.

CPUslowedbydevice.EX:CPUhasacharactertoprint- printerisbusy,soCPUmustwait.DatamaybecomingintoCPUbeforetheCPUisreadytodealwithit.

Solution:Abuffer

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Foroutputdevices- wanttheCPUtoputdatainabufferassoonasit’sready(don’twaitforthedevicetonolongerbebusy).

Thesesimplerelationshiparecalledproducer/consumer.

ProduceData

BufferConsumeData

Solution:ABuffer

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Forinputdevices- wantthedevicetoplacedatainabufferasit’sproduced.ThentheCPUcangrabitwhentheCPUisready.

Thesesimplerelationshiparecalledproducer/consumer.

ProduceData

BufferConsumeData

CircularBuffers

• Spaceisfiniteandwedonothavedynamicmemorymanagement(i.e.malloc/freeornew/delete).

• Allocateafixedsizeinmemory,then“wraparound”• ManagedwithaFirstinFirstOut(FIFO)policy.

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Circularbuffers• 2pointersmanagebufferallocationanddeallocation

• IN:pointstonextavailablelocation.Producerputsdatahere.• OUT: pointstonextdataiteminFIFOorder.Consumertakesdatafromhere.• Wearegoingtomanagethebufferwithnoothermeta-data.

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IN OUT

CircularBuffers

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INOUT

Emptycondition:IN==OUT

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Example:BufferSize =8

CircularBuffers

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OUT

Contentinbuffer

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Example:BufferSize =8

A B

C

D

CircularBuffers

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IN

OUT

BufferFull:(IN+1)modBufferSize ==OUT

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Example:BufferSize =8

A B

C

D

F

G

Why?

Can the buffer be full when OUT points to index 3(A) Yes(B) No(C) I really can’t tell

E

CircularBufferAlgorithms• Producerprocess

1.Checkstateofbuffer2.IfbufferisFULL,waitforspace3.Else,placecharacterinbufferatlocationpointedtobyIN4.IncrementINpointermoduloBUFSIZE5.Exit

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Spin-waitwhenbufferisfull

CircularBufferAlgorithms• ConsumerProcess

1.Checkstateofbuffer2.IfbufferisEMPTY,waitfordata3.Else,getdatafromlocationpointedtobyOUTpointer4.IncrementOUTpointermoduloBUFSIZE5.Exit

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Spin-waitwhenbufferisempty