Scheduling Doubt

Question

WHEN A PROCESS GOES FOR IO(INPUT/OUTPUT), A DIFFERENT PROCESS IS SCHEDULED TO A PROCESSOR. DOES IT HAPPEN IN ALL THE THREE IO MODES- INTERRUPT DRIVEN, PROGRAMMED IO AND DMA?? PLEASE CAN ANYONE TELL THIS IN DEPTH.

Comments

Dear mam, I m not getting ur doubt. In the sense when the one process requires to perform some I/O then in this mean time we are going to pick another process and give it to CPU in order to maintain degree of multiprogramming. It is fine but How are u inter-relating this thing to different Data transfer modes.

Data transfer modes are nothing but with the help of these modes we are going to transfer the data of I/O devices like HDD, Mouse to memory or copy the data from I/O devices to main memory with the help of CPU or DMA...

plz make me understand........

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Actually in Programmed IO CPU is involved during the entire IO, so i was thinking how could scheduler give the CPU to another process when the first process is doing IO. Its something troubling my mind and may be my doubt is purely silly but again its better to sound silly than to be in any confusion :)

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@ sushmita does it not mean scheduler is give time through different scheduling.

like RR when preempted, otherwise FCFS, SJF.

rt?

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If I m getting u then ur doubt is in case of single processor system,when one process is doing I/O and doing I/O means transferring the data from I/O device to main memory with the help of CPU. when CPU has already involved in data transfer with programmed I/O , so during this mean time How can we execute or schedule another process..

Am I right??????

Answer 1

Well, that is a really nice question. Let me try to explain as much as I know.

Whenever a process needs to transfer some data from / to an input device, such a operation is known as I/O operation.

• If the process (of transfer of data from / to device) is performed under the control a program, that is it sets the registers of input / output device, then such a mode is called Programmed I/O. In such a mode, the CPU initiates the (driver) program, to transfer the bytes from / to device. And it waits for the operation to complete. However, the program does not tell CPU that the operation has completed. CPU has to query the status of (certain) registers to get the information. So, CPU has to continously Poll to get the status. This severly degrades the performance of system. 

In short CPU waits while program completes IO.

• If the process (of transfer of data from / to device) is performed in such a way that after the transfer is complete , the program interrupts CPU to tell that the action is completed, then such a mode is called Interrupt driven I/O. In such case, CPU issues the command for transfer and continues to work on something else. During this time, IO module gets data form device and once it gets, it interrupts CPU. On getting interrupt, CPU transfers data to memory.

In short, CPU is able to do other things while IO in progress

• If the process (of transfer of data from / to device) is performed in such a way that after the transfer is carried out entirely to memory by device other than CPU, it is called DMA mode. In such a mode, CPU tells DMA that it needs some data from IO device. And then CPU continues doing something else. DMA, interrupts CPU, to get a bus (for memory) to transfer data to / from memory from / to IO. CPU grants bus and then does something else.

In short, CPU is able to do other things here also while IO is in progress. In this mode, CPU performs good.

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