Modes of transfer in computer

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Introduction

• Binary information received from an external device is usually stored in memory.

• Information transferred from the central computer into an external device also is originally from the memory.

• Data transfer between the central computer and input and output devices may be handled in a variety of modes

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Different modes of data transfer

Programmed I/O

Interrupt initiated I/O

Direct memory access (DMA)

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Programmed I/O• These operations are a results of I/O instructions written in the

computer program. Data transfer is initiated by an instruction in

the program.

• Usually the data transfer data between CPU register and

peripheral device. Other instructions are used to transfer data

transfer data between CPU and memory.

• The peripheral has to be constantly monitored. Once a data

transfer is initiated, the CPU is required to monitor the interface

to see when a transfer can again be made.www.ustudy.in

Applications of programmed I/O method

Useful in small low speed computers.

Used in systems that are dedicated to monitor a

device continuously.

Used in the data register.

Used to check the status of the flag bit and branch.

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Interrupt initiated I/O• This can be avoided by using an interrupt facility and special

commands to inform the interface to issue an interrupt

request signal when the data are available from the device.

• Meanwhile, CPU can proceed to execute another program.

The interfaces keeps monitoring the device.

• When the interface determines that the device is ready for

data transfer, it generates an interrupt request to the

computer.www.ustudy.in

Service routines of Interrupt initiated I/O

Service routines of interrupt initiated I/O can be

chosen in two ways.

Vectored interrupt

Non-vectored interrupt

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Direct Memory Access (DMA)• The interface transfer data into and out of the memory unit

through the memory bus.

• The CPU initiates the transfer of supplying the interface with the

starting address and the number of words needed to be

transferred and then proceed to execute other tasks.

• When the request is granted by the memory controller, the DMA

transfer the data directly into memory. The CPU delays its

memory access operation to allow the direct memory I/O

transfer. www.ustudy.in

Direct Memory Access (DMA)

During DMA transfer, the CPU is idle and has no

control of the memory buses. The buses can be

disabled by using two special control signals.

Bus request (BR)

Bus Grant (BG)

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I/O Processor

Many computers combines the interface logic with the

requirements for direct memory access into one unit and

call it an I/O processor. The IOP can handle many

peripherals through a DMA and interrupt facility. The

computer is divided into three separate modules in such a

system.

Memory unit

CPU

IOPwww.ustudy.in

I/O Processor

CPU is the master while the IOP is a slave processor. The

CPU performs the tasks of initiating all operations.

The operations include

Starting an I/O transfer

Testing I/O status conditions needed for making

decisions on various I/O activites.

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Reference Link

• http://www.mans.edu.eg/faceng/english/computers/PDFS/PDF4/4.4.pdf

• www.authorstream.com

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The End

…..Thank you…..

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