Computer Science Previous Year Questions (page 18)

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Perverse software is software written to cause harm, i.e. malicious software (malware) such as viruses, worms and trojans.

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High-level languages are machine-independent, so programs are portable, and their readable English-like syntax makes errors easier to find and debug. Both are features.

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Cloud computing offers on-demand self-service, rapid scalability (elasticity) and a pay-per-use (measured service) billing model, so all three are key features.

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Apache HTTP Server is open-source software. PHP is also free and open source, but the official key accepts only Apache (Only II).

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Binary search needs a sorted list with direct (random) access to the middle element, which an array provides. A linked list has no direct access to the middle, so III is false.

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A matrix with a high proportion of zero entries is a sparse matrix; one with few zeros is a dense matrix.

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volatile is a C keyword (type qualifier). Automatic, Integer and Constant are not keywords; the C keywords are auto, int and const.

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Modifying val twice in one expression is undefined behaviour in C, but common compilers (e.g. GCC) apply both increments first, making val = 3, and then compute 3 * 3 = 9.

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A binary tree of height n has at most 1 + 2 + ... + 2ⁿ = 2ⁿ⁺¹ − 1 nodes (completely full) and at least n + 1 nodes (one node per level, a chain).

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^ binds tighter, so 3 ^ 4 is grouped first: 9 v (3 ^ 4) v 2. v is left associative, so it is (9 v (3 ^ 4)) v 2: root v with left child the inner v and right child 2.

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Full-duplex (duplex) mode lets both stations transmit at the same time. Half duplex allows both directions but only one at a time, and simplex is one direction only.

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BCD codes each decimal digit in 4 bits. 1000 has 4 digits, so 4 × 4 = 16 bits are needed.

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(100001)₂ = 33 and (110)₂ = 6. 33 ÷ 6 gives quotient 5 = (101)₂, remainder 3.

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EDVAC stands for Electronic Discrete Variable Automatic Computer, a stored-program machine designed by Eckert, Mauchly and von Neumann.

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Second-generation computers (about 1956-1963) used transistors in place of the vacuum tubes of the first generation. The third generation used ICs.

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The control unit directs and coordinates all parts of the computer, but it does no actual data processing itself; arithmetic and logic are done by the ALU. Both statements are true.

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In the Roman system a symbol has the same value wherever it appears (X is always 10), so it is non-positional. Decimal, octal and binary are positional.

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In series, resistances add: 4 + 4 = 8 ohm.

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The output is 0 only when both inputs are 0 and 1 otherwise, which is the OR function (A + B).

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In a half adder, sum = A ⊕ B and carry = A·B (AB), because a carry is produced only when both inputs are 1.

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In RTL the transistors are driven into saturation, so it is a saturated logic family. ECL keeps the transistors out of saturation, so it is non-saturated (and very fast). Both pairs are correct.

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m select lines can choose one of 2^m inputs, so a multiplexer with n inputs needs 2^m = n (e.g. 8 inputs need 3 select lines).

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With J and K tied together (J = K = T), the JK flip-flop holds when T = 0 and toggles when T = 1, which is a T flip-flop.

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A NAND gate with both inputs at A gives (A·A)' = A', so it acts as a NOT gate.

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Shadow paging is a recovery technique that needs no log: it keeps a shadow page table unchanged and a current page table for updates, and commits by switching tables.

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In an ER diagram an ellipse is an attribute and a diamond is a relationship. In crow's foot notation a circle with a bar means optional one, and a double bar means mandatory one.

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A timestamp is a unique value the DBMS gives each transaction when it starts, used to order transactions in timestamp-based concurrency control.

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The timestamp-ordering variants are total, partial and multiversion timestamp ordering. Multi-level timestamp ordering is not one of them.

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Data allocation is the process of deciding which sites of a distributed database will store each fragment or copy of the data.

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Data fragmentation splits a database (horizontally, vertically or mixed) into logical units called fragments, which can be stored at different sites.

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>= (greater than or equal to) is a comparison operator. + and / are arithmetic operators. = is also used for comparison in SQL, but the key takes >=.

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A POS expression is an AND of OR terms, which maps to a two-level NOR-NOR circuit. NOR is a universal gate, so POS is implemented with NOR gates.

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A D flip-flop copies its input on the clock edge: Q becomes 1 when D = 1 and goes to the 0 (reset) state when D = 0.

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R2' + 1 is the 2's complement of R2, so R1 + R2' + 1 adds R1 to the 2's complement of R2, which is R1 − R2, and stores it in R3.

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Micro-operations are classified into four types: register transfer, arithmetic, logic and shift micro-operations.

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One mode bit (the I bit) is enough: I = 0 for a direct address and I = 1 for an indirect address.

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A wireframe is a basic layout of a website that shows what content goes on each page and how the pages link (navigation between them). Both are true.

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MTTF means Mean Time To Failure, the average time a system runs before it fails. It is a common software reliability metric.

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The waterfall model is also called the linear sequential model (or classic life-cycle model) because its phases follow one after another.

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RAD (Rapid Application Development) is a high-speed version of the linear sequential model that gets fast delivery through component-based construction.

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Project estimation techniques are empirical, heuristic and analytical. Prototyping is a development model, not an estimation technique.

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A PERT chart is a network of boxes (or nodes) and arrows. The boxes are activities and the arrows show the dependencies between them, so both statements are true.

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In data dictionary notation, { } means iteration (repetition), ( ) means optional data and [ , ] means selection of one of the alternatives.

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Cohesion measures the functional strength of a module, i.e. how closely the elements inside one module work toward a single task. High cohesion is desirable.

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Java has no free-standing functions: every method must belong to a class, and member functions are called methods. Both are correct.

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Each test increments m and decrements k. The loop stops when m >= k, which first happens at m = 2000, k = 2000 (1000 iterations). So 2000 is printed.

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Rotational delay (latency) is the time spent waiting for the desired sector to rotate under the read/write head. Seek time is moving the head to the right track.

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A DMA controller moves data directly between I/O devices and main memory without passing each word through the CPU.

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The key fits a need of 3 drives per process: deadlock is impossible if n(3 − 1) + 1 ≤ 7, so n = 3. (If each needed only 2 drives as written, up to 6 processes would be safe.)

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Free memory = 500 − (100 + 130) = 270 KB. With compaction, all of it can be joined into one block, so any request up to 270 KB succeeds and the smallest request that can be denied is 271 KB.