GATE 2026 CS1 Q.16: solution and answer

Official answer: BSource: Official answer key, GATE 2026 CS1 (IIT Guwahati)

1. A data hazard occurs when overlapping (pipelined) execution of instructions can change the outcome relative to strictly sequential execution, due to a dependency through a shared register operand.

2. Read-after-read (RAR): both instructions only read the register. Reads don't change the register's value, so the order in which they read it doesn't matter — no hazard.

3. Write-after-read (WAR) and write-after-write (WAW): in a simple single-issue, in-order pipeline, every instruction reads its operands (ID stage) and writes its result (WB stage) strictly in program order, so a later instruction can never overtake an earlier one's read or write. WAR/WAW can only cause trouble under out-of-order or overlapped-write execution, not in this standard in-order pipeline — so they are not the answer here.

4. Read-after-write (RAW): a later instruction needs to read a register that an earlier instruction has not yet finished computing/writing. If the read (in ID) happens before the earlier instruction's write (in WB) has completed, the pipeline reads a stale value — producing a wrong result unless stalled or the value is forwarded. This is a genuine data hazard even in a simple in-order pipeline.

5. Since RAW is the only one of the four dependencies that can actually corrupt a result under ordinary in-order pipelined execution, it is the dependency that causes a data hazard.

6. Hence the answer is Read-after-write, option (B).