OS Concepts - Practice MCQs for CCAT

Correct Answer: B - Priority Scheduling

Priority Scheduling can cause starvation - solved using aging.

Correct Answer: D - LRU

LRU (Least Recently Used) is a page replacement algorithm, not CPU scheduling.

Correct Answer: B - All four conditions hold simultaneously

Deadlock needs: Mutual Exclusion, Hold & Wait, No Preemption, Circular Wait.

Correct Answer: A - Deadlock avoidance

Banker's algorithm avoids deadlock by checking safe states.

Correct Answer: B - Process synchronization

Semaphores synchronize concurrent processes to avoid race conditions.

Correct Answer: D - Page is not in memory

Page fault occurs when a required page is not in main memory.

Correct Answer: A - Excessive paging activity

Thrashing: system spends more time paging than executing.

Correct Answer: D - Disk as extension of RAM

Virtual memory uses disk space to extend available memory.

Correct Answer: D - Saving and loading process states

Context switch saves current process state and loads another.

Correct Answer: A - I/O or event

Blocked process waits for I/O completion or an event.

Correct Answer: D - Mutex is binary, Semaphore can have count

Mutex is binary (locked/unlocked), Semaphore can have count > 1 for multiple resources.

Correct Answer: A - Replaces oldest page

FIFO replaces the oldest page in memory (first in, first out).

Correct Answer: B - Least recently used page

LRU replaces the page that has not been used for the longest time.

Correct Answer: C - Paging

Internal fragmentation occurs in paging when allocated memory is larger than needed.

Correct Answer: D - Variable/dynamic partitioning

External fragmentation occurs in variable partitioning when free memory is scattered.

Correct Answer: C - FIFO

Belady's anomaly: FIFO may have more page faults with more frames.

Correct Answer: D - Kernel has full access, User restricted

Kernel mode has full hardware access; User mode is restricted for protection.

Correct Answer: B - Request OS services

System calls are the interface between user programs and operating system services.

Correct Answer: D - Semaphores for synchronization

Producer-Consumer uses semaphores (empty, full, mutex) for proper synchronization.

Correct Answer: A - Response time and context switches

Smaller quantum = better response time but more context switches; larger = opposite.

Correct Answer: C - Compiler design

Compiler design is not a function of an operating system. An OS is responsible for memory management, process management, file management, I/O management, and security.

Correct Answer: C - Hard real-time OS

Hard real-time OS guarantees that critical tasks are completed within strict time deadlines, which is essential for missile guidance systems where any delay could be catastrophic.

Correct Answer: B - To provide an interface between a process and the operating system

System calls provide a programmatic interface between a running process and the operating system. They allow user-level processes to request services from the kernel such as file operations, process control, and communication.

Correct Answer: C - Priority Scheduling

In Priority Scheduling, a low-priority process may wait indefinitely if higher-priority processes keep arriving. This problem is known as starvation and can be solved using aging.

Correct Answer: A - Time quantum / time slice

Time-sharing OS uses a time quantum (time slice) to allocate CPU time to each user process in a round-robin fashion, ensuring fair and interactive use of the system.

Correct Answer: C - fork()

The fork() system call creates a new child process that is a duplicate of the calling (parent) process. exec() is used to replace the process image, wait() waits for child termination, and exit() terminates a process.

Correct Answer: A - To give control of CPU to the process selected by the scheduler

The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler. It involves context switching, switching to user mode, and jumping to the appropriate location in the program.

Correct Answer: B - Short processes wait behind a long process in FCFS

The convoy effect occurs in First Come First Serve (FCFS) scheduling when a long CPU-burst process holds the CPU, causing shorter processes to wait for a long time, leading to lower CPU and device utilization.

Correct Answer: B - Round Robin

Round Robin is a preemptive scheduling algorithm where each process is assigned a fixed time quantum. If a process doesn't complete within its quantum, it is preempted and placed at the end of the ready queue.

Correct Answer: B - Increased CPU utilization

Multiprogramming increases CPU utilization by keeping multiple processes in memory. When one process waits for I/O, the CPU can switch to another process, reducing CPU idle time.

Correct Answer: D - A process is an instance of a program in execution

A program is a passive entity (code stored on disk), while a process is an active entity — an instance of a program that is currently being executed, with its own program counter, stack, and data section.

Correct Answer: D - 0

In most operating systems, a mode bit value of 0 indicates kernel mode (supervisor mode), while a value of 1 indicates user mode. This hardware-supported mechanism protects the OS from errant user programs.

Correct Answer: D - Excessive paging activity causing system slowdown

Thrashing occurs when a system spends more time swapping pages in and out of memory than executing processes. It happens when the degree of multiprogramming is too high and processes don't have enough frames.

Correct Answer: C - Compiler Operating System

There is no such thing as a 'Compiler Operating System.' A compiler is a translator program, not an OS type. Valid OS types include Batch, Time-sharing, Distributed, Real-Time, and Network OS.

Correct Answer: B - To overlap I/O of one job with computation of another

Spooling (Simultaneous Peripheral Operations Online) uses a disk as a buffer to overlap I/O operations of one job with the computation of another, improving overall system throughput.

Correct Answer: A - Expected CPU burst time

SJF scheduling selects the process with the shortest expected CPU burst time next. It is optimal in terms of average waiting time but requires knowing or estimating burst times in advance.

Correct Answer: C - To signal the CPU that an event needs attention

An interrupt is a signal to the CPU indicating that an event requires immediate attention. When an interrupt occurs, the CPU saves its state and transfers control to an interrupt handler routine.

Correct Answer: A - Only essential services run in kernel space

In a microkernel architecture, only the most essential services (like IPC, basic scheduling, and memory management) run in kernel space. Other services like file systems and device drivers run in user space.

Correct Answer: C - Replaces the current process image with a new program

The exec() system call replaces the current process's memory image with a new program. The process ID remains the same, but the code, data, and stack are replaced with the new program.

Correct Answer: A - Saving and restoring the state of a process so execution can be resumed later

Context switching involves saving the state (context) of the currently running process and restoring the state of the next process to be executed. This includes saving/restoring registers, program counter, and memory maps.

Correct Answer: A - 7.0

SJF order is 3, 6, 7, 8. Waiting times are 0, 3, 9, and 16. Average waiting time = (0+3+9+16)/4 = 7.0.

Correct Answer: C - The first program that runs when a computer is powered on

The bootstrap program (bootloader) is the first program that runs when a computer is powered on or restarted. It is stored in ROM/EEPROM and is responsible for loading the operating system kernel into memory.

Correct Answer: C - To allow devices to transfer data to/from memory without CPU intervention

DMA allows hardware devices to transfer data directly to/from memory without involving the CPU for each byte. This frees the CPU to perform other tasks during bulk data transfers.

Correct Answer: A - SJF

Shortest Job First (SJF) is provably optimal for minimizing the average waiting time among all non-preemptive scheduling algorithms. Its preemptive version (SRTF) is optimal among all algorithms.

Correct Answer: B - 2

With burst time 10 and quantum 4: First run uses 4 (remaining 6, preempted), second run uses 4 (remaining 2, preempted), third run uses 2 (completes). So the process is preempted 2 times.

Correct Answer: C - A kernel where all OS services run in kernel space as a single program

A monolithic kernel runs all OS services (file system, device drivers, memory management, etc.) in kernel space as a single large program. Examples include Linux and traditional Unix kernels.

Correct Answer: A - Selects processes from job pool to ready queue

The long-term scheduler (job scheduler) selects processes from the job pool (secondary storage) and loads them into memory (ready queue). It controls the degree of multiprogramming.

Correct Answer: B - VxWorks

VxWorks is a widely used real-time operating system (RTOS) designed for embedded systems that require deterministic response times. Windows, Ubuntu, and macOS are general-purpose operating systems.

Correct Answer: A - Time from submission of a request to the first response

Response time is the time from when a request (process) is submitted until the first response is produced. It is an important metric in interactive systems and is different from turnaround time.

Correct Answer: D - In preemptive, a running process can be interrupted; in non-preemptive, it runs to completion

In preemptive scheduling, the OS can interrupt a running process and allocate the CPU to another process. In non-preemptive scheduling, a process holds the CPU until it finishes or voluntarily yields.