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Input/Output

I/O Modules and Peripheral Devices

I/O modulesWide variety of peripherals

Delivering different amounts of dataAt different speedsIn different formatsPerformance mismatch with processor/memory

InterfaceBetween processor/memory and peripheral devices

Generic Model of I/O Module

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I/O Modules and Peripheral Devices

I/O modulesPeripheral devices

Provide a means of exchanging data betweenProvide a means of exchanging data between external environment and computerDifferent categories

Human readable, machine readable, communication

InterfaceControl, data, and status

External Device Block Diagram

Typical I/O Steps

CPU checks I/O module device statusI/O module returns statusIf ready, CPU requests data transferI/O module gets data from deviceI/O module transfers data to CPU

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I/O Module Diagram

Three Types of I/O

Programmed I/OInterrupt-Driven I/ODirect Memory AccessDirect Memory Access

Programmed I/O

CPU has direct control over I/OSensing statusRead/write commandsRead/write commandsTransferring data

Memory mapped and Isolated I/O

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Memory Mapped I/O vsIsolated I/O

Memory-mapped I/OControlling devices just like reading/writing memoryRequires no special instructions

l d (S l ) /OIsolated (Special-purpose) I/ONo loss of memory addresses to peripheralsSimpler address decoding logicCan be fasterSpecial I/O instructions needed

Interrupt Driven I/O

Overcomes CPU waitingNo repeated CPU checking of devicep gI/O module interrupts when ready

Interrupt Driven I/O Basic Operation

Interrupt request signal (device processor)Finishes execution of the current instructionAcknowledgement of interruptSave PC and program state informationTransfer control to interrupt service handlerRestore PC and program state informationExecute next instruction

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Direct Memory Access

Why DMA? Interrupt driven and programmed I/O require active CPU intervention

T f t i li it dTransfer rate is limitedCPU is tied up

DMA is the answerWhat is DMA (direct memory access)

An efficient technique to move large volume of data

How DMA ?

Additional I/O module on system busDMA controller takes over bus from CPU

CPU is suspended if requiring buses

DMA Operation

CPU tells DMA controller:Read/WriteDevice addressDevice addressStarting address of memory block for dataAmount of data to be transferred

CPU carries on with other workDMA controller deals with transferDMA controller sends interrupt when finished

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Data Transfer Using DMA

Cycle StealThe DMA controller generates read or write signals at cycles when the processor is not g y pusing the bus

Burst TransferTransfer a block of data within certain consecutive cyclesCPU may be halted completely.

ExampleConsider a device that can transfer data at rate of 8KB/s. Assume the device is driven by interrupt and the interrupt takes total 100 us for transferring one byte

If the processor has to be interrupted for every byte, what fraction of processor time is consumed by this device?

Now assume that the device has two 16-byte buffers and interrupts the processor when one of the buffers is full. The interrupt will take 8us more for each additional byte. How much processor time is consumed by this device?

Summary

I/O interface and devicesThree types of I/O

Program I/OProgram I/OInterrupt I/ODMA