Question 1

Operating systems — command interpreters: What is the primary advantage when a command processor executes only built-in (internal) commands, instead of launching external user-defined commands?

Accepted Answer

Introduction / Context: Many operating systems include a command processor (also called a shell) that accepts user commands. Some commands are implemented internally (built-ins) while others are external programs. This question focuses on the performance implications of using only built-in commands versus invoking external executables. Given Data / Assumptions: The command processor can execute built-in commands directly without loading a separate process. External commands require locating a program file, loading it from storage, and creating a new process context. We are comparing overall execution speed and overhead. Concept / Approach: Built-in commands run within the shell process itself. This avoids disk I/O, executable image loading, dynamic linking, and process creation. Eliminating these steps reduces latency for short utility operations such as directory navigation, variable manipulation, or quick configuration changes. Step-by-Step Solution: Identify overhead for external commands: path search + disk read + load/relocate + create process + context switch.Contrast with built-ins: parse and execute directly within the shell, reusing the existing process context.Conclude that minimizing process launches and disk access yields faster command turnaround.Therefore, the dominant advantage is higher execution speed. Verification / Alternative check: Empirical shell timing (for example, time of a built-in echo or cd) is consistently faster than launching an external program of similar function because of the avoided process creation and I/O. Why Other Options Are Wrong: Flexibility via batch files: Batch scripts work with both built-ins and externals; not unique to built-ins. Common command set across hardware: Standardization depends on the OS distribution, not on being built-in. Users can create system programs: That relates to external commands, not built-ins only. None of the above: Incorrect because the speed benefit is real and primary. Common Pitfalls: Assuming built-ins are always preferable; sometimes external tools are richer in features. Also, security policies and maintainability may favor external tools despite overhead. Final Answer: the processing is much faster than would otherwise be the case when user-defined commands are used

Question 2

Magnetic disks — track organization: On a rotating magnetic disk, each concentric track is subdivided into which smaller addressable units?

Accepted Answer

Introduction / Context: Understanding how magnetic disks organize data physically is essential for topics such as file systems, disk scheduling, and performance tuning. Tracks, sectors, and blocks are foundational terms that map the physical medium to logical storage. Given Data / Assumptions: A disk surface contains concentric circular tracks. Each track is further divided for addressable data storage. Modern disks may also use zone bit recording, but the basic addressable unit remains consistent. Concept / Approach: The smallest addressable physical unit on a track is a sector. Traditionally, sectors store a fixed number of bytes (commonly 512 bytes or 4096 bytes in advanced format drives). File systems build larger logical blocks on top of physical sectors for efficiency and alignment. Step-by-Step Solution: Identify the hierarchical structure: platter → surface → track → sector.Recall that controllers read/write integral sectors, not arbitrary byte ranges.Therefore, the subdivision of a track into addressable units is called sectors. Verification / Alternative check: Disk specifications and low-level formatting utilities reference CHS (cylinders, heads, sectors), confirming sectors are the canonical subdivision of tracks. Why Other Options Are Wrong: Periods / groups: Not standard magnetic disk terms for track subdivision. Zones: In zone bit recording, a zone is a collection of tracks with the same sectors-per-track, not the basic subdivision of a single track. None of the above: Incorrect because sectors is correct. Common Pitfalls: Confusing logical blocks with physical sectors; assuming all tracks have the same number of sectors (not necessarily true in zoned recording). Final Answer: sectors

Question 3

I/O priority control — daisy chaining: Which of the following is NOT a characteristic of a daisy-chain priority control scheme for multiple devices?

Accepted Answer

Introduction / Context: Daisy-chain priority is a classic hardware scheme for servicing interrupts or bus requests from multiple peripherals. The devices are physically chained so that request/grant signals propagate in order, giving a fixed hardware priority from the head to the tail of the chain. Given Data / Assumptions: Devices are connected in a linear chain for request/grant propagation. Signals pass through each device to the next, establishing priority by physical position. We are identifying a statement that does not reflect this scheme’s characteristics. Concept / Approach: In daisy chaining, priority is inherently positional and therefore fixed by wiring order, not dynamically programmable. The wiring can simplify control-line count since a single grant line can be passed device-to-device. However, a device failure can block downstream devices, a known drawback. Step-by-Step Solution: List properties: fixed, position-based priority; simple wiring; potential single-point failures along the chain.Evaluate options: programmability contradicts the fixed priority nature.Conclude that “priority is programmable” is not a characteristic. Verification / Alternative check: Bus standards and textbooks describe daisy chains as having static priority without additional logic; programmable priority requires separate controllers or software schemes. Why Other Options Are Wrong: Easy to add devices: Generally true by extending the chain. Failure of one device may affect others: True—signals may not propagate past a failed device. Control lines independent of device count: Largely true; the same grant chain is reused. None of the above: Incorrect because one option is indeed not a characteristic. Common Pitfalls: Assuming “programmable” priority just because firmware exists; ignoring the reliability trade-off of chained propagation. Final Answer: priority is programmable

Question 4

Language processors — definition: An assembler is best described as which of the following types of programs?

Accepted Answer

Introduction / Context: Language processors transform source code into forms the machine can execute. Assemblers, compilers, linkers, loaders, and interpreters serve distinct roles. Correctly distinguishing them is essential for systems programming and toolchain comprehension. Given Data / Assumptions: Assembly language is a low-level, mnemonic representation of machine instructions. Machine language is the binary encoding executed by the CPU. Different tools exist for high-level languages and for loading/executing code. Concept / Approach: An assembler maps mnemonics and operands into opcodes and machine addresses, producing object code (and often symbol tables and relocation info). Compilers target high-level languages; interpreters simulate execution; loaders place programs into memory for execution. Therefore, the statement that an assembler automates translation from assembly to machine code is correct. Step-by-Step Solution: Match each definition to the correct term: assembler vs compiler vs loader vs interpreter.Identify that assembly → machine translation is the assembler’s job.Eliminate definitions that refer to high-level languages or loading/executing. Verification / Alternative check: Toolchains show “as” (assembler) producing object files from .s/.asm sources, distinct from compilers (cc) and linkers (ld). Why Other Options Are Wrong: Places programs into memory: This is a loader. Accepts high-level language: This is a compiler. Appears to execute source directly: This is an interpreter. None of the above: Incorrect because the assembler definition is present. Common Pitfalls: Confusing assembling with compiling; overlooking that linkers combine multiple object files after assembling/compiling. Final Answer: a program that automate the translation of assembly language into machine language.

Question 5

Operating systems — caching: What is the principal purpose of block or buffer caches in a general-purpose operating system?

Accepted Answer

Introduction / Context: Buffer caches (also called page cache or block cache) are core OS components that keep recently accessed disk blocks in RAM. They bridge the performance gap between slow storage devices and much faster main memory and CPU. Given Data / Assumptions: Disks have higher latency and lower throughput than RAM. Temporal and spatial locality means data is often reused shortly after it is read. The OS manages a cache of blocks/pages to exploit locality. Concept / Approach: By caching frequently accessed blocks, the OS serves subsequent reads from RAM and coalesces or defers writes (write-back/write-through policies). This reduces average access time and alleviates contention on storage devices, improving system responsiveness and throughput. Step-by-Step Solution: Recognize that caches store recent disk blocks in memory.Understand that cache hits avoid expensive disk I/O.Conclude that the main goal is to improve disk performance perceived by applications. Verification / Alternative check: System monitoring (e.g., cache hit ratios) demonstrates reduced I/O operations and improved performance when caches are effective. Why Other Options Are Wrong: Handle interrupts: Managed by the interrupt subsystem, not the buffer cache. Increase capacity of main memory: Caches use existing memory; they do not increase capacity. Speed up main memory reads: RAM access is already fast; caches target slow disk I/O. None of the above: Incorrect because improving disk performance is precise. Common Pitfalls: Confusing buffer cache with virtual memory paging; ignoring write policies and data consistency issues. Final Answer: to improve disk performance

Question 6

Backup strategies — definitions: An incremental backup is best described as which of the following?

Accepted Answer

Introduction / Context: Backup schemes determine what data is copied and when. Understanding full, differential, and incremental backups is essential for designing reliable recovery plans while managing storage and time costs. Given Data / Assumptions: Files have metadata indicating last modification time and backup status. A full backup captures everything; follow-up backups may capture only changes. Goal is to minimize backup duration and storage while preserving recoverability. Concept / Approach: An incremental backup copies only files changed since the most recent backup of any kind (often the last incremental or full). This reduces data volume and time but may require a longer restore sequence: latest full + all subsequent incrementals in order. Step-by-Step Solution: Identify backup types: full, differential, incremental.Match definition: incremental = changes since last backup.Conclude that “saves only files that have recently changed” is accurate. Verification / Alternative check: Backup software documentation consistently describes incrementals as capturing changes since the previous backup, contrasting with differential (since last full) and full (everything). Why Other Options Are Wrong: Should be done each month: Schedule is policy-dependent, not definitional. Uses more tapes: Incrementals typically use less storage per run; restore may need more sets. Saves all files: That is a full backup, not incremental. None of the above: Incorrect because option D is correct. Common Pitfalls: Confusing incremental with differential; underestimating restore complexity when many incremental sets exist. Final Answer: saves only files that have recently changed

Question 7

Disk access timing: The total time to prepare and retrieve a block for reading from a magnetic disk is composed of which elements?

Accepted Answer

Introduction / Context: Disk performance analysis relies on understanding the separate contributors to delay: seek time, rotational latency, and transfer (transmission) time. Many definitions distinguish between the time to position the head (prepare) and the time to actually transfer data once positioned. Given Data / Assumptions: Seek time moves the read/write head radially to the target track. Rotational latency is the wait for the desired sector to rotate under the head. Transmission (transfer) time is the actual data movement once the head is over the sector. Concept / Approach: If the question emphasizes “prepare a disk drive mechanism for a block of data to be read,” the preparation is complete when the head is in position and the correct sector is under the head. That comprises seek time + rotational latency. Transfer time begins after preparation, during the actual reading of the block. Step-by-Step Solution: Identify preparation steps: position to track (seek) + align sector (latency).Exclude transfer, which occurs after positioning is complete.Therefore, preparation time equals seek time + latency. Verification / Alternative check: Storage texts define access time as seek + latency, and total service time as access time + transfer time. The wording “prepare the mechanism” maps to access time. Why Other Options Are Wrong: Latency alone: Ignores track seek. Latency + transmission: Missing seek, and includes transfer. Latency + seek + transmission: That is total service time, not just preparation. None of the above: Incorrect because seek + latency is correct. Common Pitfalls: Using “access time” and “transfer time” interchangeably; forgetting that large sequential reads amortize seek/latency over many blocks. Final Answer: latency plus seek time

Question 8

File design — record sizing: How should a file’s record length be selected for robust performance and storage efficiency?

Accepted Answer

Introduction / Context: File record length impacts storage overhead, access speed, and flexibility. The right choice depends on the nature of the data, access patterns, and update behavior in databases, log files, and structured binary formats. Given Data / Assumptions: Some workloads store homogeneous, fixed-size entries. Other workloads store records with variable-length fields or sparse content. Access patterns (random vs sequential) influence optimal layout. Concept / Approach: There is no one-size-fits-all rule. Fixed-length records favor constant-time indexing and simple offsets; variable-length records reduce internal fragmentation for heterogeneous data. Designers weigh update frequency, typical record size distribution, and the need for direct addressing. Step-by-Step Solution: Determine data characteristics: field sizes, optional fields, typical growth.If records are uniform, fixed length may maximize simplicity and speed.If records vary widely, variable length minimizes wasted space and improves packing.Therefore, match record length strategy to data characteristics. Verification / Alternative check: Database storage engines (heap files, slotted pages) and log-structured systems adopt designs based on observed data distributions, confirming the adaptive approach. Why Other Options Are Wrong: Always fixed / always variable: Absolutist and ignores workload diversity. Depends upon file size: File size alone is not the key determinant. None of the above: Incorrect because matching to data is correct. Common Pitfalls: Ignoring alignment and page size effects; neglecting update patterns that lead to fragmentation; overlooking the cost of pointers/offset tables for variable records. Final Answer: should be chosen to match the data characteristics

Question 9

Compiler pipeline — phases: Which of the following are standard phases in a compilation process?

Accepted Answer

Introduction / Context: Compilers transform source programs into target code through a series of well-defined phases. Recognizing these phases helps developers understand diagnostics, optimization opportunities, and tool behavior. Given Data / Assumptions: Front end phases include lexical analysis, syntax analysis, and semantic checks. Back end phases include optimization and code generation. Terminology may vary, but core functions are common across compilers. Concept / Approach: Lexical analysis tokenizes character streams into tokens. Code generation emits target code (machine code or intermediate forms). While “static analysis” is sometimes a broader term for compile-time checks, the question asks which listed items are compilation phases, and both lexical analysis and code generation are canonical phases. Step-by-Step Solution: Identify standard phases: lexical → syntax → semantic → optimization → code generation.Confirm that both lexical analysis and code generation belong to this sequence.Select “Both of the above”. Verification / Alternative check: Compiler textbooks outline these phases explicitly, with examples from widely used compilers such as GCC and LLVM. Why Other Options Are Wrong: Lexical analysis alone / Code generation alone: Incomplete. Static analysis: Often overlaps semantic checks but the option as a stand-alone choice here is used as a distractor. None of the above: False, as both are valid phases. Common Pitfalls: Confusing lexical analysis with parsing; equating static analysis tools (linters) with compiler internals, though they share techniques. Final Answer: Both of the above

Question 10

Marketing term — software bundling: In personal computing, what does the practice of "bundling" typically refer to?

Accepted Answer

Introduction / Context: Vendors often package additional items with a computer to increase perceived value, differentiate offerings, or encourage adoption of a software ecosystem. This practice is widely called bundling in retail and OEM channels. Given Data / Assumptions: Computers are sold with preinstalled operating systems and applications. The added software may be licensed at low or no apparent cost to the buyer. The term focuses on inclusion with purchase, not separate sale. Concept / Approach: Bundling describes including software (and sometimes services) with a hardware purchase at no additional line-item cost, typically preinstalled or provided as redemption codes. The strategy increases usability out of the box and can promote specific applications or platforms. Step-by-Step Solution: Identify that bundling means inclusion alongside the primary item.In PCs, the most common bundle is software shipped with the machine.Therefore, “giving away software with a computer purchase” captures the intended meaning. Verification / Alternative check: Retail listings often advertise “bundled software” such as office suites, antivirus trials, or media editors preinstalled on new computers. Why Other Options Are Wrong: Selling computers alone: Opposite of bundling. Selling peripherals with the computer: That is a hardware bundle, but the common textbook use here refers to software inclusion; option D is the more precise match. Selling software to run on computers: That is normal retail sale, not bundling with purchase. None of the above: Incorrect because software bundling is well known. Common Pitfalls: Confusing promotional software bundles with trialware; assuming bundled items are free of licensing terms—often they are time-limited or OEM-licensed. Final Answer: giving away software with a computer purchase

Question 11

Preemptive CPU scheduling Which of the following is an example of a preemptive scheduling algorithm used by operating systems?

Accepted Answer

Introduction / Context: Preemptive CPU scheduling allows the operating system to interrupt a running process and assign the CPU to another process, improving responsiveness and fairness. This question asks you to identify a classic preemptive algorithm from common scheduling strategies. Given Data / Assumptions: The options list well known algorithms: round-robin, shortest-job-next, and priority-based. “Preemptive” means a running task can be paused by the scheduler before it voluntarily yields. We assume standard textbook definitions (shortest-job-next is non-preemptive; shortest-remaining-time is the preemptive variant). Concept / Approach: Round-robin scheduling assigns a fixed time quantum to each ready process in a cyclic order. When the quantum expires, the scheduler preempts the process and dispatches the next one. This is the quintessential example of preemption designed for time-sharing systems. Step-by-Step Solution: Identify which listed algorithms are inherently preemptive.Round-robin uses a time quantum and forced preemption at the quantum boundary.Shortest-job-next (also called shortest-job-first) is typically non-preemptive; its preemptive cousin is shortest-remaining-time-first.Priority-based scheduling can be either preemptive or non-preemptive depending on implementation, so it is not guaranteed preemptive.Therefore, select Round-robin. Verification / Alternative check: Any system configured with a time slice (quantum) and context switching on timer interrupts implements preemption; this matches round-robin behavior. Why Other Options Are Wrong: b: Shortest-job-next is the non-preemptive form; it waits for the running job to finish. c: Priority-based may be preemptive or non-preemptive; the option as stated is ambiguous. d: Not all listed algorithms are preemptive. e: Incorrect because round-robin is preemptive. Common Pitfalls: Confusing shortest-job-next with shortest-remaining-time-first; assuming all priority schedulers preempt lower priorities—this depends on design. Final Answer: Round-robin

Question 12

Storage technologies and access time Which device type has the highest (slowest) access time among the following?

Accepted Answer

Introduction / Context: Access time is the delay between a read/write request and the beginning of data transfer. Understanding relative access times across storage tiers is foundational for performance tuning and systems design. Given Data / Assumptions: Cache resides close to the CPU and is orders of magnitude faster than disks. Magnetic disks (hard drives) have mechanical latency and seek time but are faster than legacy floppy disks. “Swapping devices” commonly refer to disk-based backing store used for paging; typical latency is similar to or better than floppy disks due to higher rotation speed and density. Concept / Approach: Compare typical latencies: cache in nanoseconds; hard disks in milliseconds; floppy disks also in milliseconds but with slower rotation speed and longer seek/settle times, yielding higher (worse) access times than hard disks. Therefore, among the listed options, floppy disks are slowest. Step-by-Step Solution: Rank by physical characteristics: no moving parts (cache) vs. spinning media (disks).Recognize that floppy disks rotate at lower RPM (for example, 300 RPM) and have lower data density than modern hard drives.Swapping devices are typically hard disks or SSDs; a floppy is not used for swapping in practical systems.Select the device with the greatest access delay: floppy disk. Verification / Alternative check: Historical metrics: cache tens of nanoseconds; HDD seek + rotational latency ~ 5–15 ms; 3.5 inch floppy average access ~ 100–300 ms. The floppy is clearly slowest. Why Other Options Are Wrong: b: Cache has the lowest access time. c: Swapping devices are usually hard disks or SSDs, faster than floppies. d: Hard disks are faster than floppies due to higher RPM and better mechanics. Common Pitfalls: Assuming “swapping devices” means something slower than floppies; in real computing environments, floppies are not used for virtual memory. Final Answer: Floppy disk

Question 13

Addressing modes in computer architecture In which addressing mode is the memory address of the operand given explicitly within the instruction itself?

Accepted Answer

Introduction / Context: Addressing modes define how instructions reference their operands. Recognizing the differences helps with assembly programming, compiler design, and understanding generated machine code. Given Data / Assumptions: We compare absolute (direct) addressing against immediate and indexed addressing. “Explicitly given address” means the instruction contains the target memory address as an operand field. Terminology: absolute mode is also called direct mode in many texts. Concept / Approach: Absolute addressing places the effective memory address directly in the instruction. Immediate mode embeds the data value, not an address. Index mode combines a base (often from a register) with an offset to compute the effective address at run time. Step-by-Step Solution: Interpret the phrase “address of the location of the operand is given explicitly”.Match that phrase to absolute (direct) addressing where EA = address field.Eliminate immediate mode (EA not used; operand is a literal) and index mode (EA computed as base + index).Select Absolute mode. Verification / Alternative check: Assembly examples often show instructions like “LOAD A, 1000h” using direct addressing where 1000h is the memory address; this is absolute mode. Why Other Options Are Wrong: b: Immediate places the operand value in the instruction, not its address. c: Index computes an address from components at run time, not a single explicit address. d: “Modulus mode” is not a standard addressing mode. Common Pitfalls: Confusing “immediate value” with “address given immediately”; the former is data, not a pointer. Final Answer: Absolute mode

Question 14

Role of a base register table What does a base register table track during program loading and execution?

Accepted Answer

Introduction / Context: Base registers support relocation by holding a starting address for a segment, while instruction displacements address locations relative to that base. A base register table is a loader/linker structure that manages which registers serve this role and with what values. Given Data / Assumptions: We are discussing system-level structures used by loaders/OS on architectures with base/displacement addressing. The table must record register availability and their base contents. Other options describe unrelated OS concepts or programmer-facing features. Concept / Approach: During relocation, the loader assigns base values to registers and keeps metadata about which registers are bound as base registers. The base register table is the natural place for this bookkeeping, enabling correct address translation at run time through base + displacement addressing. Step-by-Step Solution: Identify what information a loader needs: register identity and base addresses.Match this to the option that explicitly mentions both the registers in use and their base values.Eliminate distractors that refer to programmer syntax (a), memory partitioning policy (c), or generic OS terms (d).Choose option b. Verification / Alternative check: Classic mainframe and some minicomputer OS manuals define base register tables exactly this way: a mapping of base registers to segment bases for relocation and protection. Why Other Options Are Wrong: a: Programmers can reference labels; base registers are handled by the system, not typically hard-coded. c: Multiprogramming is orthogonal to base register tables. d: “Control programs” is not the meaning of base register table. Common Pitfalls: Confusing a symbol table with a base register table; mixing logical segmentation with fixed partitions. Final Answer: Which general purpose registers are in use as base registers and the base addresses they hold

Question 15

Capacity planning in systems management Which statement best characterizes capacity planning practices?

Accepted Answer

Introduction / Context: Capacity planning ensures computing resources meet current and future workload demands. Planners forecast CPU, memory, storage, and network needs based on measured utilization and growth trends. Given Data / Assumptions: Performance metrics such as CPU utilization, I/O wait, queue lengths, throughput, and response time are available. Tools and the operating system influence what can be measured and controlled. Capacity planning applies to environments of all sizes. Concept / Approach: Accurate capacity planning relies on detailed measurements and historical data to model workloads. OS instrumentation and monitoring tools provide these data points. The process is not OS-independent; features and limits differ by platform. Even small installations benefit from planning to avoid outages and overspending. Step-by-Step Solution: Identify the option that reflects a universal requirement: detailed performance information.Note that platform specifics and tool availability matter, invalidating claims of independence.Recognize that small sites still require right-sized resources.Select option a as the accurate description. Verification / Alternative check: Industry frameworks (for example, capacity management in ITIL) emphasize measurement and monitoring as prerequisites for forecasting, confirming option a. Why Other Options Are Wrong: b: OS features and schedulers affect performance behavior. c: Without monitoring tools, reliable data are unavailable. d: Small installations also need planning to avoid resource saturation. Common Pitfalls: Relying on averages and ignoring peaks; neglecting storage IOPS or network bottlenecks; failing to account for seasonal growth. Final Answer: Requires detailed system performance information

Question 16

Nassi–Shneiderman (structured) charts Which statement about Nassi–Shneiderman charts is NOT true?

Accepted Answer

Introduction / Context: Nassi–Shneiderman (NS) charts, also known as structured flowcharts, depict program logic using nested boxes for sequence, selection, and iteration. They enforce structured programming principles by construction. Given Data / Assumptions: NS charts are indeed graphical design tools. They support sequence, selection (including multiway), and iteration. CASE constructs are a form of multiway selection. Concept / Approach: In NS charts, selection is represented using branched boxes; multiway (CASE) selection can be drawn as stacked alternatives within the selection construct. Therefore the claim that they cannot represent CASE constructs is false, making it the “NOT true” statement to select. Step-by-Step Solution: Confirm that NS charts are graphical design tools (true).Confirm that NS charts support sequence, selection, and iteration (true).Evaluate the statement about CASE: NS charts can represent multiway branches (so the statement is false).Pick the “NOT true” statement: option b. Verification / Alternative check: Standard software engineering texts provide NS chart templates for IF, WHILE, REPEAT, and CASE; this directly contradicts option b. Why Other Options Are Wrong: a: Correct—NS charts are a structured design tool. c: Correct—NS charts represent sequence, selection, iteration. d: Can’t be right because a and c are true while b is not. Common Pitfalls: Confusing classic flowcharts (with arbitrary jumps) with NS charts (structured, no arbitrary flow lines). Final Answer: These charts cannot represent CASE (multiway selection) constructs.

Question 17

Command interpreter basics Which statement correctly describes a command interpreter (shell) in an operating system?

Accepted Answer

Introduction / Context: The command interpreter, often called a shell, is the component that reads user commands, interprets them, and invokes programs. Examples include bash, zsh, PowerShell, and the classic DOS command processor. Given Data / Assumptions: We consider typical desktop and server operating systems. Shells accept free-form commands with flexible syntax and options. Graphical shells exist, but the term “command interpreter” generally refers to a textual interface. Concept / Approach: In multiuser systems, the shell is the default interactive interface when a user logs into a terminal. It parses commands, expands variables and wildcards, and launches processes. While some environments provide menus, a command interpreter does not inherently require fixed formats or menu navigation. Step-by-Step Solution: Identify the role of a shell: the primary textual interface for command entry.Reject claims about fixed formatting; shells support various syntaxes, quoting, and pipelines.Reject “menu driven” as a defining property; that describes UI shells or programs, not the core command interpreter.Select option a as the accurate, general statement. Verification / Alternative check: On Unix-like systems, logging into a console yields a shell prompt (for example, bash) which is the primary interface; Windows PowerShell similarly acts as a command interpreter. Why Other Options Are Wrong: b: Commands are not strictly fixed format; options and flags provide flexibility. c: Menus are a different UI paradigm. d: The phrase “SCL interpreter” is context-specific and not a general distinction for shells. Common Pitfalls: Assuming a GUI file manager is the same as a command interpreter; conflating scripting languages with shell semantics. Final Answer: It is usually the primary user interface.

Question 18

Critical region in concurrent programming What best defines a critical region in process synchronization?

Accepted Answer

Introduction / Context: In concurrent programming, multiple threads or processes may need to access shared data. The area of code that reads or writes shared resources is called a critical region (or critical section). Correct synchronization prevents race conditions. Given Data / Assumptions: Shared resources include variables, files, devices, or data structures. Mutual exclusion mechanisms are used to protect the critical region. Semaphore operations (P and V) are one implementation, but not the definition itself. Concept / Approach: A critical region is defined functionally by its access to shared resources, requiring mutual exclusion to avoid interleaving hazards. Various primitives enforce this: locks, semaphores, monitors, or atomic instructions. Therefore, the definition centers on “where shared resources are accessed”. Step-by-Step Solution: Identify the common property of all critical regions: shared state access.Recognize that P/V (wait/signal) are one way to guard such code but are not mandatory in the definition.Eliminate options that refer to bugs or crashes; those are symptoms, not definitions.Select option c. Verification / Alternative check: Operating systems and concurrency texts define a critical section as any code that accesses shared variables needing mutual exclusion—a direct match to option c. Why Other Options Are Wrong: a and b: Not definitional; a well-implemented critical region may be bug-free. d: P/V describe one guarding technique; other mechanisms exist. Common Pitfalls: Assuming that only semaphores define critical sections; forgetting that I/O devices are shared resources too. Final Answer: A program segment where shared resources are accessed

Question 19

Preemptive vs non-preemptive scheduling facts Identify the false statement about CPU scheduling approaches.

Accepted Answer

Introduction / Context: CPU scheduling affects responsiveness, predictability, and deadline satisfaction. Preemptive schedulers can interrupt running tasks; non-preemptive schedulers let tasks run until they block or finish. This item asks which statement is false. Given Data / Assumptions: Time-sharing systems emphasize responsiveness and fairness. Real-time systems emphasize determinism and meeting deadlines. Preemption allows higher priority or time-critical tasks to run immediately. Concept / Approach: Real-time operating systems typically use preemptive, priority-driven schedulers so that higher-priority tasks can preempt lower-priority ones to meet deadlines. Therefore, the claim that real-time systems “generally use non-preemptive” scheduling is false. Step-by-Step Solution: Assess each statement against standard OS behavior.a: Correct definition of preemption.b: True—preemption enables good response in time-sharing.c: True—preemption tends to bound response time variability.d: False—real-time systems generally rely on preemption to enforce priorities. Verification / Alternative check: RTOS documentation (for example, priority-based preemptive schedulers in QNX, VxWorks, FreeRTOS) confirms heavy reliance on preemption. Why Other Options Are Wrong: a, b, c align with standard definitions and practice. e is incorrect because there is a false statement (d). Common Pitfalls: Confusing “non-preemptive real-time” with cooperative systems; while possible, it is not the general practice for hard real-time guarantees. Final Answer: Real-time systems generally use non-preemptive CPU scheduling.

Question 20

Compiler data structures What is the Uniform Symbol Table in a compiler front end?

Accepted Answer

Introduction / Context: Compilers maintain several tables during analysis: the uniform symbol table (UST), literal table, identifier table, and terminal table. Distinguishing these helps clarify the roles of lexical and syntax analysis. Given Data / Assumptions: Lexical analysis recognizes tokens and records them for parsing. Different tables hold different categories of information (tokens, literals, identifiers, terminals). Uniform symbol table typically provides a uniform representation for tokens passed to the parser. Concept / Approach: The UST is the stream or structure of uniform symbols (tokens) produced by the scanner. Each entry commonly references supporting tables (identifier, literal) and encodes token class plus attributes. The parser consumes the UST to build syntax structures. Step-by-Step Solution: Identify which option describes a token sequence produced by lexical analysis.Option c explicitly states that the list is created by lexical analysis and used for syntax/interpretation.Map other options to their correct tables: literals (a), grammar rules or parse tables (b), terminals/keywords (d).Select option c for the UST. Verification / Alternative check: Compiler construction texts depict the interface between scanner and parser as a sequence of uniform symbols—precisely the UST concept. Why Other Options Are Wrong: a: That is the literal table. b: Refers to parsing tables/grammar; not the UST. d: Describes the terminal table. Common Pitfalls: Confusing the UST with a symbol table of identifiers only; the UST is a generalized token stream referencing other tables as needed. Final Answer: A list of tokens, in full or in part, as they appear in the program; created by lexical analysis and used by syntax analysis and later stages.

Operating Systems Concepts Questions