Cross-topic traps and rapid-fire drill

Look back at the questions that have appeared throughout this course. Very few of them were "define X".

Almost all of them were "what is the difference between X and Y?"

That is not an accident. Pairs are how interviewers test whether two ideas are separate in your head or blurred together.

Why & what

The nine pairs. For each, the answer is not two definitions side by side. It is the one thing that distinguishes them.

• Starvation vs deadlock. Could run but never chosen, vs cannot run ever. Aging fixes one; nothing fixes the other on its own. (M3 / M5)
• Mode switch vs context switch. Same process at a new privilege level, vs a different process entirely. 2 µs vs 5 µs. (M1 / M2)
• Page fault vs segmentation fault. Your page, not loaded yet, vs never your address at all. One is routine; the other kills the process. (M7)
• Internal vs external fragmentation. Waste inside a block, vs waste between blocks. Paging trades the second for the first. (M6)
• Buffer vs cache. Data on its way through, vs a copy kept in case it is wanted again. (M8)
• Concurrency vs parallelism. In progress at once, vs executing at the same instant. One core can do the first and never the second. (M2)
• Semaphore vs mutex. Anyone may signal a semaphore; only the owner may unlock a mutex. (M4)
• Logical vs physical address. What the program says, vs where it lands. (M6)
• Unsafe state vs deadlocked state. Deadlock is reachable, vs deadlock has happened. (M5)

The pattern in the answers. Notice what makes each distinction sharp. It is never a longer definition — it is a question like:

• What would fix it? (starvation vs deadlock)
• What changed? (mode vs context switch)
• What happens next? (page fault vs segfault)

Answer with the consequence, not the category.

How it works

Drilling these properly:

1. Cover the right-hand side of each pair and say the distinguishing line from memory.
2. Say it as a consequence, not a definition. "One can be fixed by aging" beats "one is a scheduling problem".
3. Attach a number where one exists. 2 µs versus 5 µs makes the mode/context pair concrete instantly.
4. Find the module for each. If you cannot place it, that module needs rereading more than this list does.
5. Then do the trace from Topic 9.1 out loud, end to end, without looking. If you can do that and these nine pairs, you are ready.

Common confusion

"These pairs are just vocabulary." Each one marks a place where two genuinely different mechanisms have similar names. Getting page fault and segmentation fault backwards is not a wording slip — it means saying that the routine mechanism kills the process.

"If I know the definitions I know the difference." Two correct definitions recited side by side is the answer that scores worst, because it does not show which one applies in a given situation. The distinguishing consequence is the answer.

Interview angle

The rapid-fire round below is how these get asked in practice: short, in any order, with no warm-up. Work through them without looking anything up.