Computer Network Questions and Answers

Answer: B

Correct answer: 47

Each hexadecimal digit carries a place value that is a power of 16, read from the right: \( 16^{0} = 1 \) and \( 16^{1} = 16 \). In 2F the digit F stands for 15, so \[ 2 \times 16 + 15 = 32 + 15 = 47. \] The letters run A = 10, B = 11, C = 12, D = 13, E = 14, F = 15, and it is worth memorising them, since almost every hexadecimal question depends on the conversion. A quicker route for small values goes through binary. Each hexadecimal digit is four bits: 2 is 0010 and F is 1111, giving 00101111, which is \( 32 + 8 + 4 + 2 + 1 = 47 \). Conversion by grouping is faster than arithmetic once the sixteen four-bit patterns are familiar. The largest two-digit hexadecimal number is FF, which is 255 - the maximum value of one byte, and the reason colour codes and byte values are so often written this way.

Answer: A

Correct answer: \( \overline{A + B} = \bar{A} \cdot \bar{B} \)

De Morgan’s first theorem states that the complement of a sum is the product of the complements: \[ \overline{A + B} = \bar{A} \cdot \bar{B}. \] In words, NOT (A OR B) is the same as (NOT A) AND (NOT B). The second theorem is its dual: \[ \overline{A \cdot B} = \bar{A} + \bar{B}. \] A truth table confirms the first at once. Taking A and B as 0,0 gives \( \overline{0} = 1 \) on the left and \( 1 \cdot 1 = 1 \) on the right; every other combination gives 0 on both sides. The practical rule for applying either theorem is: break the bar, change the sign. Split the overbar, and swap AND for OR or OR for AND. The theorems matter because they allow any circuit to be rebuilt using only NAND gates or only NOR gates, which is why those two are called universal.

Answer: D

Correct answer: Shift register

A shift register is sequential, because it is built from flip-flops and its output depends on the bits already stored as well as on the present input. The other three are combinational: their outputs are determined entirely by their present inputs, with no memory of what came before. Each can be fully specified by a truth table. The dividing line is memory. Combinational circuits include adders, multiplexers, demultiplexers, decoders, encoders, comparators and code converters. Sequential circuits include flip-flops, latches, registers, shift registers and counters, and they need a clock. Sequential circuits are described by state tables and state diagrams, and they divide further into synchronous designs, where every element is clocked together, and asynchronous ones, where changes propagate as they occur. Every sequential circuit contains combinational logic inside it, generating the next state and the outputs, so the two categories are layered rather than opposed.

Answer: D

Correct answer: ALU

The arithmetic logic unit carries out arithmetic operations such as addition and subtraction and logical operations such as AND, OR, NOT and XOR, together with shifts and comparisons. It is one of the three parts of the central processing unit, the others being the control unit, which fetches and decodes instructions and directs the rest of the machine, and the registers, which hold the operands and results the ALU works on. Alongside its result the ALU sets flag bits - zero, carry, sign and overflow - and those flags are what conditional branch instructions test, which is how a program makes decisions. RAM and ROM are memory and store data rather than operate on it, and a bus is a set of parallel conductors carrying data, addresses or control signals between components. Subtraction inside the ALU is performed as addition of the 2’s complement, which is why no separate subtractor is needed.

Answer: A

Correct answer: Its output can be logic 0, logic 1, or a high-impedance state

A tri-state buffer has an ordinary data input, an output, and an enable control. When enabled it passes the input through; when disabled its output enters a high-impedance state, effectively disconnecting it from the wire. That third state is not a logic level. It is the electrical condition of being neither driving high nor driving low, and it is what makes shared buses possible. Many devices can be wired to the same set of conductors provided that only one is enabled at a time; the rest float and do not interfere. Without it, two outputs driving the same line at opposite levels would create a short circuit from supply to ground - called bus contention - which wastes power and can damage the devices. The high-impedance state is written Z in truth tables and simulation. The alternative arrangement for a shared line is the open-collector or open-drain output with a pull-up resistor.

Answer: C

Correct answer: a small, very fast storage location inside the processor

A register is a small storage location built directly into the processor, typically holding one word - 32 or 64 bits - and accessible in a single cycle. Registers are the fastest storage in the machine and sit at the top of the memory hierarchy, above cache, main memory and disk. They are few in number precisely because they are expensive in silicon area, and a large part of what a compiler does is decide which values are worth keeping in them. Several have fixed roles. The program counter holds the address of the next instruction; the instruction register holds the instruction being decoded; the accumulator holds arithmetic results in simpler architectures; the memory address and memory buffer registers interface with memory; the stack pointer tracks the top of the stack; and the status or flag register holds the zero, carry, sign and overflow bits that conditional branches test. General-purpose registers hold whatever the program needs.

Answer: B

Correct answer: dc supply voltage and the average supply current

Power dissipation is

\[ P_D = V_{CC}\times I_{CC(avg)} \]

The average current is used because the current drawn varies as the gate switches; the peak value would overstate the power, and the supply is dc, not ac.

Answer: A

Correct answer: 0010 0101

In binary coded decimal each decimal digit is encoded separately in four bits. So 25 becomes 2 and 5 encoded individually: 2 is 0010 and 5 is 0101, giving 0010 0101. This is not the same as the pure binary value of 25, which is 11001 - the second option, and the usual trap. BCD deliberately sacrifices compactness for ease of conversion. Because only ten of the sixteen four-bit patterns are used, the six combinations from 1010 to 1111 are invalid in BCD, and arithmetic must correct for them: when a BCD addition produces a result above 9 or generates a carry, 6 is added to bring it back into range. The code is used wherever numbers are displayed or entered digit by digit - calculators, digital clocks, instrument panels - because converting each digit to a seven-segment display needs no division. The waste is about 20 per cent of the available codes.

Answer: B

Correct answer: 47

In hexadecimal the digit F stands for 15, so \(2F_{16} = 2 \times 16 + 15 = 47\). Hexadecimal is convenient because each digit maps onto exactly four binary bits.

Answer: A

Correct answer: less power than a TTL IC

CMOS draws significant current only while switching, so its static dissipation is minute compared with TTL, which draws current continuously through its output stage. Lowering the supply to 3 V reduces it further still, since dynamic power varies with the square of the supply voltage.