INPUTOUTPUT MODULE

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CHAPTER 7

• INPUT/OUTPUT MODULE

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Chapter Outline

• Introduction
• External devices
• I/O Module
• I/O Techniques
• Programmed I/O
• Interrupt Driven
• Direct Memory Access (DMA)

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7.1 Introduction

• External devices are NOT generally connected
  directly into the bus structure of the computer
• Wide variety of devices require different logic
  interfaces
• Mismatch of data rates
• Different data representations
• The I/O Module
• Provides a standard interface to the CPU and the
  bus
• Tailored to specific I/O device and its interface
  requirements
• Relieves the CPU of the management of the I/O
  devices

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Generic Model of I/O Module
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7.2 External Devices

• There are 3 types of external devices-
• Human-readable
• To communicate with users e.g .
• Machine-readable
• To communicate with the computer devices e.g
• Communication
• As interface to the remote devices e.g ..

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• Interface to I/O module form of
• Control signals
• Status signals
• Data bits

Figure 1 Block Diagram of External Device
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7.3 Input/Output Module

• I/O modules have 5 major functions -
• Control timing
• Coordinate the flow of traffic between internal
  resources external devices
• Processor communication
• Communicate with the CPU
• E.g .
• Device Communication
• To communicate status information, commands and
  data with devices

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• Data buffering
• To buffer data coming from MM to match the rate
  of the peripheral
• Error detection
• To detect and report errors to the processor
• E.g paper jam, bad disk track

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Figure 2 Block Diagram of an I/O Module
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7.4 Input /Output Techniques

• 3 commonly used techniques are-
• Programmed I/O
• Interrupt Driven
• Direct Memory Access (DMA)

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7.4.1 Programmed I/O

• I/O operation in which CPU issues the I/O command
  to the I/O module
• CPU is in direct control of the operation
• CPU waits until the I/O operation is completed
  before it can perform other tasks
• Completion is indicated by a change in the module
  status bits
• CPU must periodically poll the module to check
  its status

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• As a result of the speed difference between CPU
  and the peripheral devices, programmed I/O wastes
  an enormous amount of CPU processing power
• Very inefficient
• CPU is slowed to the speed of the peripheral
• Advantages-
• Simple to implement
• Requires very little special software or hardware

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7.4.2 Interrupt-driven I/O

• To reduce the time spent on I/O operations, the
  CPU can use an interrupt-driven approach
• CPU issues I/O command to the module
• CPU continues with its other tasks while the
  module perform its task
• Module signals CPU when I/O operation is finished
  (the interrupt)

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• CPU responds to the interrupt by executing an
  interrupt service routine (ISR) and then
  continues on with its primary task
• CPU recognizes and responds to interrupts at the
  end of an instruction execution cycle

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7.4.3 Direct Memory Access

• Both programmed and interrupt-driven I/O require
  the continued involvement of the CPU in ongoing
  I/O operations
• Direct memory accessing takes the CPU out of the
  task except for initialization of the operation
• Large amount of data can be transferred between
  MM and the devices without severely impacting CPU
  performance

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• CPU initializes the DMA module
• R/W operation defined
• I/O device involved
• Starting address of memory block
• Number of words to be transferred
• CPU continues with other work

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• DMA operates by stealing bus cycles from the
  CPU
• Uses the bus when CPU is not using it no impact
  on the CPU performance
• Accesses memory to retrieve a data word
• Forwards the word to the I/O devices

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DMA Configurations (1)

• Single Bus, Detached DMA controller
• Each transfer uses bus twice
• I/O to DMA then DMA to memory
• CPU is suspended twice

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DMA Configurations (2)

• Single Bus, Integrated DMA controller
• Controller may support gt1 device
• Each transfer uses bus once
• DMA to memory
• CPU is suspended once

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DMA Configurations (3)

• Separate I/O Bus
• Bus supports all DMA enabled devices
• Each transfer uses bus once
• DMA to memory
• CPU is suspended once