In an operating system process model, when a process is waiting for some event to occur (for example an I O completion), in which state is that process considered to be?

Introduction / Context:
Operating systems describe process life cycles using states such as new, ready, running, blocked and terminated. Understanding these states is essential for analysing scheduling behaviour and performance. A common exam question asks about the state of a process when it is waiting for an external event, such as completion of an I O operation or arrival of a signal.

Given Data / Assumptions:

We are using a typical process state model with ready, running and blocked states.
The process has issued an operation that cannot complete immediately, such as a disk read or waiting on a synchronization object.
Until the event occurs, the process cannot continue executing its instructions.
The processor is free to run other ready processes during this time.

Concept / Approach:
When a process requests an operation that will take time, the operating system moves it from the running or ready state to a blocked or waiting state. In this state, the process is not eligible to run on the CPU. The scheduler considers only processes in the ready state for execution. Once the event occurs, an interrupt or signal causes the operating system to move the process back to the ready state, where it will eventually run again.

Step-by-Step Solution:
Step 1: Consider a running process that issues a system call to read from disk. Step 2: The operating system initiates the I O operation and then marks the process as blocked, since the process cannot proceed until data arrives. Step 3: The CPU is then given to another ready process so that the system remains productive. Step 4: When the disk operation completes, an interrupt notifies the operating system, which changes the process state from blocked back to ready. Step 5: At no point during the wait is the process in the running state; it is clearly in the blocked or waiting state.

Verification / Alternative check:
Standard process state diagrams in operating system texts label the waiting or blocked state as the one where the process is waiting for some event. The running state is reserved for the process currently using the CPU, while the ready state is for those that could run but are not currently running. The terminated state is for processes that have finished execution and will not run again.

Why Other Options Are Wrong:
In the running state, the process is actively executing on the CPU, which is not possible while waiting for I O completion.
In the ready state, the process is able to run but is not currently running; a process that needs an event cannot be considered ready.
The terminated state means the process has completed and exited; a process waiting for an event is still active and will resume later.

Common Pitfalls:
Some learners confuse ready and blocked states, thinking that any non running process is ready. The key difference is whether the process could run if the CPU were free. Another pitfall is to assume that a blocked process is wasting CPU time; the whole point of blocking is to allow other processes to use the CPU while one waits for slower external events.

Final Answer:
When a process is waiting for an event such as I O completion, it is in the blocked or waiting state, not in the running or ready state.