I/O Organization

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I/O Organization

• Accessing I/O• The important parts of any computer system

are, CPU, Memory & I/O devices (peripherals)• CPU fetches instructions (opcodes & operands)

from memory, processes them and stores results in memory

• Other components called I/O system• The main function of I/O system to transfer

information between CPU or memory and outside world

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I/O Organization

• Accessing I/O• I/O devices cannot be connected directly to the system bus• Because• Different I/O with different methods of operation, so it would

be impractical to incorporate to control a range of devices• Data transfer rate of peripherals is often much slower than

that of memory / CPU, so it would be impractical to use high speed system bus to communicate

• Different I/O with different data formats and word length that of CPU used

• To overcome all these difficulties, it is necessary to use a module in between system bus and I/O devices called I/O module or I/O system or I/O interface

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I/O Organization

• Accessing I/O• Single bus structure to connect I/O

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I/O Organization

• Accessing I/O• Single-bus structure• The bus enables all the devices connected to it

to exchange information• Typically, the bus consists of three sets of lines

used to carry address, data, and control signals• Each I/O device is assigned a unique set of

addresses

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I/O Organization

• Accessing I/O• The data bus lines are used to receive / send

data from or to the I/O• The address bus lines selects the I/O device for

data transfer • The control bus lines the direction of data flow

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I/O Organization

• Accessing I/O• I/O devices are accessed through I/O interface• The major parts of I/O interface are• 1 Control & Timing• 2 CPU Communication• 3 Device Communication• 4 Data Buffering• 5 Error Detection

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I/O Organization

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I/O Organization

• Accessing I/O• Control & Timing To coordinate the flow of traffic b/w

internal resources (memory, system bus) and external devices• CPU Communication involves different types of signal

transfers such as– Processor sends commands to the I/O, over control bus – Exchanges of data b/w processor & I/O, over data bus– Data transfer rate of I/O is often slower than the processor. So it is

necessary to check the peripheral is ready or not for data transfer. If not processor must wait

– No of peripheral devices may connected to the I/O interface. The I/O controls the communication of each peripherals with processor. So it must recognize one unique address of each peripheral

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I/O Organization

• Accessing I/O• Data buffering Data transfer rate of each

peripheral devices are quite high than that of processor & memory

• The data coming from memory/processor are sent to I/O interface, buffered in I/O and sent to the peripheral device at its data rate

• Data are buffered in I/O interface so as not to tie up the memory in a slow transfer operation.

• I/O interface can able to operate at both peripheral & memory speed

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I/O Organization

• Accessing I/O• I/O interface is also responsible for error

detection and reporting errors to the processor• Type of errors are• Mechanical• Electrical malfunctions• Bad disk• Transmission error etc

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I/O Organization

• Accessing I/O• I/O interface consists of data register, status/control

register, address decoder and external device interface logic.• Data register holds the data being transferred to or from the

processor• Status/control register contains information relevant to the

operation of I/O device• Both data register & Status/control register connected to

data Bus• Address line drive the address decoder• Address decoder enables the device to recognize its address

when address appears on the address lines.

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