Operating systems

Question

Blocked

A process transitions to a blocked state when it cannot carry on without an external change in state or event occurring. For example, a process may block on a call to an I/O device such as a printer, if the printer is not available.

Source : https://en.m.wikipedia.org/wiki/Process_state

My questions is Let say process p is in running state .Now it wants printer access and if printer is available so process should start transfering data to printer for printing.So now while doing in this in which state process will be

 

Answer 1

The process will be in blocked state. The term "blocked" doesn't mean unavailability of resources - it just means that the job is blocked from using the CPU.

Whenever a job initiates an I/O request, it won't be using the CPU for that time. It's the job of the scheduler then to find another process which can run during the time when the former job's I/O is being serviced. When the I/O is finished, the job then signals an interrupt to the operating system and the scheduler decides what has to be done next.

EDIT: Doesn't the hardware require usage of the CPU?

No, it doesn't. Whenever the CPU issues an I/O instruction such as reading from a hard drive, the request is processed by the hardware/device controller, which is a specialised chip present on the hardware. 

"Originally these were simple chips that performed specific operations for the CPU, but modern hardware controllers are basically their own microprocessors with firmware and everything, so they are capable of very complex operations without the main CPU's help. The controller is able to read and write directly to and from system RAM using what is called a Direct Memory Access (DMA) controller, a special chip that transfers data from the hardware controller to main RAM without the CPU needing to do anything. Thus, the CPU does not need to micromanage all tasks involved with I/O. At one time this used to be the case, but these tricks (interrupts, DMA, special controllers) were invented in order to improve CPU performance and make things a lot more efficient."

Source: https://stackoverflow.com/questions/13596997/why-is-the-cpu-not-needed-to-service-i-o-requests

Comments

Yes, that's correct.

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One thing when controller is done with doing tranfer it inform to CPU that i am done.so cpu can resume that process.But here we are adding it back to ready..?

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When the I/O is finished, the controller signals to the OS that its work is done. After that, the job moves back to the ready queue and it is now the responsibility of the schedule whether to immediately schedule it or finish some other work and then run it.