GATE CS 2009 Q32 — CPU Scheduling

To determine the correct statements, let us analyze the standard process state transitions shown in the diagram:

• Transition A: $\text{Ready} \rightarrow \text{Running}$ (Dispatch)
• Transition B: $\text{Running} \rightarrow \text{Ready}$ (Preemption/Time-out)
• Transition D: $\text{Running} \rightarrow \text{Blocked}$ (I/O or Event Wait)
• Transition E: $\text{Blocked} \rightarrow \text{Ready}$ (I/O or Event Completion)

Now, let us evaluate the statements:

• Statement I is FALSE: When a process makes transition $D$ (Running $\rightarrow$ Blocked), the CPU becomes idle. Before another user process can make transition $A$ (Ready $\rightarrow$ Running), the OS scheduler must execute to select the next process. Thus, transition $A$ does not happen "immediately" without this intermediate scheduling overhead.
• Statement II is TRUE: Transition $E$ (Blocked $\rightarrow$ Ready) occurs when an asynchronous I/O operation completes. This event is handled via an interrupt and can occur while another process $P_1$ is actively executing in the $\text{Running}$ state.
• Statement III is TRUE (and IV is FALSE): The existence of transition $B$ (Running $\rightarrow$ Ready) indicates that a process can be taken off the CPU and placed back into the ready queue before it finishes or blocks. This is the defining characteristic of preemptive scheduling.

Therefore, statements II and III are TRUE.