In operating systems, what is a device queue and which processes or requests are stored in it?

Introduction / Context:
Devices such as disks, printers and network interfaces can serve only one request at a time or have limited parallelism. When multiple processes need the same device, the operating system must queue their requests. The concept of a device queue captures this idea. This question asks you to define a device queue and identify which kinds of items it stores.

Given Data / Assumptions:

• Multiple processes may request service from the same hardware device.
• The device may be busy serving one request at a time.
• Other requests must wait until the device is free.
• The operating system organises these waiting requests using queues.

Concept / Approach:
When a process issues an I O request for a device that is currently busy, the operating system places either the process or its I O request into a device queue associated with that device. Each device has its own queue or set of queues. The device driver or I O scheduler removes requests from the queue in some order, such as first come first served or an optimised algorithm. While a process is waiting in a device queue, it is usually blocked and does not consume CPU time until the I O completes.

Step-by-Step Solution:
Step 1: Distinguish between queues for CPU scheduling and queues for I O devices. Step 2: Recognise that a device queue is tied to a specific physical or logical device. Step 3: Understand that when a request arrives and the device is busy, the request or associated process is enqueued. Step 4: Note that when the device finishes a request, it takes the next one from the device queue. Step 5: Choose the option that describes a device queue as holding processes or I O requests waiting for a device to become available.

Verification / Alternative check:
Operating system diagrams often show a set of device queues connected to various devices. Processes move from the ready queue to a device queue when they request I O and from the device queue back to the ready queue after completion. This behaviour matches the correct option and is different from the job queue, ready queue or list of terminated processes described in other options.

Why Other Options Are Wrong:
Option B is really describing the job queue, which holds jobs waiting for admission to memory, not for devices. Option C describes the ready queue, which contains processes waiting for CPU time rather than for an I O device. Option D talks about terminated processes, which are no longer requesting I O and therefore would not be in any device queue.

Common Pitfalls:
Students sometimes confuse the various queues used in process management and I O scheduling. It is useful to remember that each device queue is specific to a device and stores pending input or output requests, whereas the ready queue is global for the CPU and the job queue holds jobs before they enter memory. Keeping the roles separate in your mind will make it easier to answer questions like this one.

Final Answer:
A device queue is a queue that holds processes or I O requests waiting for a particular device to become available.