CS Fundamentals interview questions and answers

The core-subject rounds common in campus hiring and still used to probe depth in experienced interviews.

17 published across 6 topics.

Computer Science FundamentalsA 22-minute read on how the topic fits together, before the questions.CS fundamentals59 short answers on one page, for revising rather than studying.

Operating Systems

3 questions

Processes and threads, scheduling, virtual memory and paging, synchronisation, and deadlock handling.

hardConcept

A service stops accepting new connections while CPU and memory look fine. Which operating-system limits would you suspect, and how would you tell them apart?

Suspect a bounded resource rather than a saturated one: file descriptors, threads, ephemeral ports or the listen queue. Each has a different scope — per process, per user, per cgroup, per system — and each announces itself with a different error, which is how you tell them apart. Use this RLIMIT answer to show the decision, trade-off, and evidence rather than a memorised definition.

5 minmid, senior, staff

mediumConcept

What actually happens during a context switch, and why should an application developer care?

A context switch in an operating system saves one thread's CPU state, runs the scheduler and restores another thread. Application developers care because excessive switches add latency through cache and TLB disruption, lock contention and oversized thread pools. It also connects scheduling to the point an interviewer is testing.

5 minentry, mid, senior

mediumConcept

What is virtual memory, and what happens when a process touches a page that is not resident?

Virtual memory gives each process a private address space and lets the MMU map virtual pages to physical frames. A page fault traps to the kernel; a minor fault maps memory cheaply, while a major fault waits for storage and can dominate performance. It also connects PAGING to the point an interviewer is testing.

4 minmid, senior

All 3 in Operating Systems

Computer Networks

3 questions

The layered model, TCP versus UDP, congestion control, routing, and what happens when you load a URL.

mediumConcept

When would you choose UDP over TCP, given that TCP is reliable and UDP is not?

TCP guarantees an ordered byte stream and pays for it with head-of-line blocking and retransmissions that arrive too late to be useful. UDP is right when a late packet is worthless, when you need multicast, or when you intend to build your own reliability with different trade-offs.

4 minmid, senior

mediumConcept

You are about to change the address a hostname points to. What does the TTL actually promise you, and why do clients keep reaching the old address anyway?

A TTL is an upper bound on how long a resolver may cache a record, not a promise about when clients will notice a change. Lowering it only takes effect after one old TTL has elapsed, negative answers are cached under a different rule, and pooled connections keep using an address they resolved hours ago.

5 minmid, senior, staff

mediumConceptDesign

Walk me through everything that happens when you type a URL into the browser and press enter.

The browser checks its caches, resolves the hostname through browser, OS and resolver caches, opens a TCP connection, completes a TLS handshake, sends the request, then parses the streamed response. QUIC fuses transport and crypto setup into one round trip and stops one lost packet stalling every stream.

6 minmid, senior, staff

All 3 in Computer Networks

DBMS Theory

3 questions

Relational algebra, functional dependencies, normal forms, and concurrency-control theory.

hardConcept

Two transactions each check a rule, then both commit changes that break it, and neither overwrote the other's row. What happened?

This is write skew. Both transactions read the same snapshot, decide independently, and write disjoint rows, so there is no write-write conflict for the engine to catch. The invariant spanned rows it never saw as related, so either make the conflict physical or use true serialisability.

5 minmid, senior, staff

mediumConcept

Take this table to third normal form, then tell me when you would deliberately denormalise — and what does ACID guarantee?

Normalisation removes redundancy so a fact is stored once; 3NF is reached when every non-key column depends on the key and nothing but the key. You denormalise knowingly, to buy read performance with write complexity. ACID guarantees far less than candidates assume, particularly its C.

5 minentry, mid, senior

hardConcept

What does it mean for a concurrent schedule to be serialisable, and how does two-phase locking guarantee it?

A schedule is serialisable when its effect matches running the same transactions one after another in some order, which is the criterion isolation is defined against. Two-phase locking achieves it by forbidding a transaction from acquiring any lock after releasing one, and the price is deadlock.

5 minmid, senior, staff

All 3 in DBMS Theory

OOP Theory

3 questions

Abstraction and the classical definitions, plus the terminology questions asked verbatim in screens.

mediumConcept

Give me a subclass that the compiler accepts but that still breaks the Liskov substitution principle. What rule does it break?

Substitutability is a claim about behaviour, and signatures carry none of it. A subclass that rejects an argument its supertype accepted has strengthened a precondition, which the compiler cannot see: the code type-checks, and it breaks callers that were written correctly against the base type.

5 minmid, senior, staff

easyConcept

What are the four pillars of OOP, and what is the difference between abstraction and encapsulation?

Encapsulation, inheritance, polymorphism and abstraction are worth stating as consequences rather than definitions - what each one buys you and what it costs. Abstraction decides what a type means to its caller; encapsulation protects how that meaning is upheld.

5 minentry, mid, senior

mediumConcept

When you call an overridden method through a base-class reference, how does the runtime know which implementation to run?

The compiler cannot know, so it emits an indirect call through a per-class table of function pointers that the object carries a reference to. The lookup itself is a couple of memory accesses; the real cost is the optimisations the compiler cannot perform when the target is not known until run time.

5 minmid, senior

All 3 in OOP Theory

Compilers & Runtimes

2 questions

Compilation phases, interpretation versus JIT, memory layout, and garbage-collection theory.

hardConcept

A method has been running fast for an hour and then gets slower, with no code change and no extra traffic. What might the runtime be doing?

A JIT compiles against a profile gathered earlier, so its fast code rests on assumptions that can be invalidated: a new receiver type at a call site, a branch taken for the first time, a class loaded that gives an interface a second implementation. The method deoptimises and recompiles from a worse profile. It also connects deoptimisation to the point an interviewer is testing.

5 minmid, senior, staff

mediumConcept

Walk me through what happens between the source you write and the instructions the CPU runs, and where does a JIT fit into that?

Source is tokenised, parsed, checked, lowered to an intermediate representation, optimised and emitted - either ahead of time to machine code or to bytecode executed by a runtime. A JIT compiles hot bytecode at run time using profiles, which is why managed programs get faster after warm-up. Use this compilers answer to show the decision, trade-off, and evidence rather than a memorised definition.

5 minmid, senior, staff

Security Fundamentals

3 questions

OWASP categories, cryptography basics, hashing versus encryption, and secure-by-default habits.

mediumConcept

You are told the application is safe from injection because all user input is escaped as it comes in. What is wrong with that?

Escaping is meaningless without a destination, and at the point input arrives you do not know which parser will consume it. Injection is a property of the sink, so the defence belongs at each sink — bound parameters, an argument vector rather than a shell string, encoding chosen by output context — not at the edge. It also connects input validation to the point an interviewer is testing.

5 minmid, senior, staff

mediumConcept

How does HTTPS establish trust, and what does a certificate prove?

A certificate binds a public key to a domain name and is signed by a chain ending at a root already in your device's trust store. It proves that whoever holds the matching private key controlled that name at issuance - nothing about the operator's honesty - and revoking it early barely works.

5 minmid, senior, staff

mediumConcept

Why do we hash passwords instead of encrypting them, and what makes a password hash different from SHA-256?

Password hashing vs encryption comes down to reversibility: encrypted passwords can be recovered with a key, while hashes are verified one way. Password storage needs salts and slow memory-hard algorithms such as Argon2, bcrypt or scrypt, not fast SHA-256.

5 minentry, mid, senior

All 3 in Security Fundamentals