Computer Science Previous Year Questions (page 6)

Explanation

Magnetic tape is the cheapest storage per bit, and also the slowest. Registers and cache are the fastest and most expensive, so they cost the most per bit.

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In binary measures, Kilo = 2¹⁰, Mega = 2²⁰, Giga = 2³⁰ and Tera = 2⁴⁰ bytes.

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The loader (the bootstrap loader) always stays in main memory, because it has to load other programs into memory before they can run. The linker and assembler are loaded only when needed.

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The prefix * + ab − cd means (a + b) * (c − d). In postfix this becomes ab+ followed by cd−, then *, which gives ab+cd−*.

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Infix-to-postfix conversion pushes operators and parentheses onto a stack. It pops them according to precedence and associativity.

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A queue is FIFO (first in, first out). Elements are added at the rear and removed from the front.

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A queue works on FIFO (first in, first out), so the element inserted first is removed first.

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A stack works on LIFO (last in, first out). Push and pop both happen at the top.

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A constructor runs automatically each time an object of the class is created, and it initialises that object. Declaring a class does not call it.

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In C++, members of a struct are public by default, and so is struct inheritance. This is the main difference from a class.

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In C++, members of a class are private by default unless an access specifier says otherwise.

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A C++ class bundles data members and member functions together into one unit, which is encapsulation.

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C++ has three access specifiers: public, private and protected.

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The scope resolution operator :: defines a member function outside its class, for example void A::show() { }.

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Inheritance lets a derived class reuse the data and functions of an existing base class without rewriting them.

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The innermost i is 30. ::i is the scope resolution operator and refers to the global i, which is 10, so the output is 30,10. Note that :: is valid in C++, not C.

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An inline function's body is substituted at the call site, so there is no function-call overhead and execution is faster. It usually makes the code bigger, not smaller.

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a << 2 shifts 5 (101) left by two bits, giving 10100 = 20, which is the same as 5 × 4. The output is FIRST20SECOND.

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Official BPSC answer key: (C). Note: most textbooks expand cout as “character output” (B).

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A program is an ordered set of instructions that tells the computer what to do. Memory and storage only hold the program.

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A computer program is an algorithm written in a programming language that the computer can run. A flowchart only represents the logic in a diagram.

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In a mesh topology, every node has a dedicated link to every other node. If one link fails, other paths still exist, so mesh is the most reliable.

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The transport layer (for example TCP) provides end-to-end, process-to-process delivery between the source and destination hosts. The network layer only routes packets hop by hop.

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Residual error rate is the transport-layer QoS parameter for the fraction of messages lost or garbled out of all messages sent in the sampling period.

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E-mail sends small amounts of data and does not need to arrive instantly, so it does not need high-speed equipment. Bulk data transfer and LAN links do.

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A bus topology connects all devices to one shared cable called the backbone.

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A LAN (local area network) covers a small area such as a room, a floor or a building. MAN covers a city and WAN covers large distances.

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An IPv4 address is 32 bits long, written as four 8-bit octets (for example 192.168.1.1). An IPv6 address is 128 bits.

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A MAC address is a 48-bit (6-byte) hardware address, usually written as 12 hex digits, for example 00:1A:2B:3C:4D:5E.

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Hacking means gaining unauthorised access to computer systems or files, often remotely through networks or terminals.

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The defining feature of a virus is that it replicates itself (clones itself) into other programs or files. Damaging data or programs is possible but not essential.

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Software piracy is copying, distributing or using software without the permission (licence) of its authors or copyright holders.

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The partition table in the MBR describes where each partition lies on the disk. If it is corrupted, the system cannot find any partition, so the whole disk becomes inaccessible.

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A foreign key is an attribute in one table that refers to the primary key of another table. It links the two tables and enforces referential integrity.

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2NF requires 1NF plus full functional dependency of every non-key attribute on the whole primary key, so there are no partial dependencies.

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A domain is the set of allowed values, or data type, for an attribute (column) in a relation.

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Degree is the number of attributes (columns) in a relation. Cardinality is the number of tuples (rows).

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UPDATE changes the values of existing rows, for example UPDATE t SET col = val WHERE .... INSERT adds rows and ALTER changes the table structure.

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SELECT retrieves (queries) data from one or more tables. DISPLAY and SHOW are not standard SQL query commands.

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ALTER TABLE changes a table's structure, for example to add, drop or modify columns. UPDATE changes data, not structure.

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LIKE matches patterns using the wildcards % (any sequence) and _ (one character). IN tests a list and BETWEEN tests a range.

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Degree (attributes) becomes 4 + 3 = 7 and cardinality (tuples) becomes 3 + 4 = 7, so both are 7.

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DROP TABLE removes the whole table, structure and data. DELETE removes only rows, and ERASE is not an SQL command.

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The program counter (PC) holds the address of the next instruction to be fetched. It is incremented after each fetch.

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The instruction register (IR) holds the instruction that is currently being decoded and executed.

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The memory address register (MAR) holds the address of the memory location being read or written. The memory buffer register (MBR) holds the data itself.

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A buffer is temporary storage that evens out the difference in data-flow speed between two devices, such as a CPU and a printer.

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In random access memory, any location can be reached in the same fixed time once its address is given. Sequential memory, such as tape, depends on position.

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Take X=4, Y=3 and add 4567 + 2345 in octal. 7+5 = 12 = 4 carry 1, 6+4+1 = 3 carry 1, 5+3+1 = 1 carry 1, 4+2+1 = 7. The sum is 7134 = (71YX)₈.

Explanation

With n bits, two's complement covers −2ⁿ⁻¹ to 2ⁿ⁻¹ − 1. For 8 bits that is −128 to +127.