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TCS · OS
Practice OS questions specifically asked in TCS interviews – ideal for online test preparation, technical rounds and final HR discussions.
Questions
27
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TCS
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Subject
OS
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Go through each question and its explanation. Use this page for targeted revision just before your TCS OS round.
Typical process states are running (using CPU), ready (waiting for CPU), waiting or blocked (waiting for I O or event). "Interrupted" is not normally listed as a standard state in the lifecycle model.
For complete preparation, combine this company + subject page with full company-wise practice and subject-wise practice. You can also explore other companies and topics from the links below.
IPC mechanisms include message passing, shared memory, pipes, sockets etc. Virtual memory is a memory management technique, not a communication mechanism between processes.
In preemptive scheduling the operating system can interrupt a running process (for example when its time slice expires or a higher priority process arrives) and switch to another process. For instance, Round Robin and priority with preemption are preemptive algorithms. Non-preemptive scheduling means once a process starts running it will continue until it finishes or blocks for I/O; the OS does not forcibly take the CPU away. First-Come-First-Served (FCFS) and non-preemptive SJF are examples. Preemptive gives better responsiveness but adds overhead from context switches; non-preemptive is simpler but may lead to worse response times and poor handling of interactive tasks.
The ready queue is the data structure or list maintained by the operating system where all processes that are ready and waiting for CPU time reside. The scheduler picks one from this queue to run next. Other queues include waiting (blocked) queue for I/O, and termination queue for completed processes.
The Least Recently Used algorithm replaces the page that has been idle longest based on past usage. It approximates the optimal algorithm and helps reduce page faults. FIFO is simpler but may cause more faults; optimal is theoretical; MRU is usually worse in general. Interviewers expect you to know the trade-offs in overhead and complexity. :contentReference[oaicite:6]{index=6}
A hard disk drive is considered a block device because data is transferred in fixed-size blocks and any block can be accessed directly. Devices like keyboard or mouse are character devices since they transfer data one character or small stream at a time.
Journaling file systems record metadata (and sometimes data) changes in a log before applying them. If a crash happens, the system can replay the journal to restore consistency quickly. This technique is widely used in modern file systems like ext4 and XFS. :contentReference[oaicite:4]{index=4}
The SCAN disk-scheduling algorithm (also called elevator algorithm) moves the disk arm in one direction servicing all pending requests until it hits the end then reverses direction. This reduces seek time by preventing arm starvation and back-and-forth movement. Understanding disk scheduling links file system and device management topics. :contentReference[oaicite:6]{index=6}
An interrupt is a signal from a device to the CPU indicating that it requires service. When the interrupt occurs, the CPU stops the current execution, saves state, and jumps to the interrupt service routine (ISR) to handle the device. In polling (or programmed I O), the CPU repeatedly checks the device’s status in a loop, which wastes CPU cycles. Interrupts allow more efficient use of the CPU since it can perform other work until a device signals. However interrupts add complexity in context switching and latency management.
Buffering is the technique where a temporary memory area (buffer) is used to hold data while it moves between two devices or between device and main memory. The purposes include: decoupling producer and consumer speeds, reducing number of I O operations, and smoothing out bursts of data. The trade-offs are: using additional memory, possible latency for filling buffer, and complexity in flushing or synchronising buffer contents. For example print spooling uses a buffer queue to accumulate print jobs before the printer handles them.
A deadlock occurs when two or more processes are unable to proceed because each is waiting for a resource held by another process in the set. None can continue. This is a fundamental concurrency issue.
Aging is a technique used in scheduling and concurrency to gradually increase the priority of a process that has been waiting for a long time so as to prevent starvation. This ensures fairness. Interviewers expect you to know both starvation risk and aging mitigation.
In an operating system the CPU can operate in at least two modes: user mode and kernel mode. In user mode applications execute with limited privileges and cannot execute critical instructions or access sensitive hardware directly. In kernel mode the OS core and trusted code run with full privileges and can manage hardware, memory, interrupt handling and system resources. This separation is important for security because it prevents user‐level code (which may be malicious or buggy) from harming the system, accessing other processes’ memory or performing privileged operations. By restricting risky operations to kernel mode the OS reduces its attack surface.
In virtualisation architectures a Type 1 (bare-metal) hypervisor runs directly on the host machine’s hardware, offering better performance and efficiency. A Type 2 hypervisor runs on a host operating system as an application layer, and then hosts guest operating systems. Knowing this distinction is a common interview point.
Resource over-commit means that the hypervisor or virtualisation platform allows virtual machines collectively to hold more CPUs, RAM or storage allocations than the underlying physical host provides, relying on the fact that not all VMs will use their maximum at the same time. This improves utilisation but also introduces risk of contention and performance degradation. Knowing this risk-vs-benefit is useful in interviews.
DMA allows a hardware controller (the DMA engine) to move data between memory and a device without continuous CPU intervention. This frees the CPU to perform other tasks while large blocks of I/O are transferred. An OS using DMA must coordinate memory buffers, interrupt service routines and bus arbitration. Understanding DMA is often asked in system-level OS interviews because it touches hardware-software boundary.
When multiple processes need I/O or device access simultaneously the OS must coordinate resource sharing. It does so by maintaining queues (ready queue, I/O queue), using device-queues, scheduling I/O requests, applying locking or semaphores to protect device registers, managing priorities, and ensuring fairness and avoidance of starvation. For example, a print spooler may queue all print requests and let the driver service them one by one. The OS might also use priority or time-slicing for devices. In interviews mention trade-offs: fairness vs throughput, device contention, prevention of starvation, and impact on CPU scheduling.
When demand-paged processes each keep triggering page faults because their combined working sets exceed available physical memory frames, the system spends more time swapping pages in and out than doing productive work — this is thrashing. Recognising thrashing is a classic advanced OS interview topic. :contentReference[oaicite:0]{index=0}
Observability in modern operating systems and platforms means having visibility into performance, reliability and resource usage at runtime so that operators can detect anomalies, diagnose faults and optimise behaviour. OS-level metrics to track include CPU utilisation, queue lengths of ready and I/O queues, memory usage, page fault rate, context-switch rate, interrupt rate, I/O latency, scheduler load, and for cloud OS also VM/container migration events, network packet drops and NUMA node imbalances. Referencing case studies of production OS helps you show applied knowledge in interviews.
CPU scheduling aims to optimise several metrics so the system runs efficiently and fairly. These goals include maximising CPU utilization (keeping the CPU as busy as possible), maximising throughput (number of processes completed per unit time), minimising turnaround time (time from submission to completion), minimising waiting time (time a process waits in the ready queue), and minimising response time (time from request submission to first response) especially in interactive systems. An operating system must balance these often-competing goals when choosing its scheduling algorithm.
A logical address is the address that a process uses in its code. It is mapped by the memory management unit to a physical address which is the actual location in RAM. This mapping enables features like virtual memory, memory protection and isolation between processes. :contentReference[oaicite:0]{index=0}
Paging divides processes and memory into fixed-sized pages and frames. A process’s pages may reside in non-contiguous frames, which reduces external fragmentation and simplifies allocation. However internal fragmentation may still happen because a page may not be fully used. Paging relies on a page table to map logical pages to physical frames. :contentReference[oaicite:2]{index=2}
A file system is the subsystem of an operating system that manages how data is stored and retrieved on storage media. It handles naming, directories, metadata, allocation and free-space management. Understanding this is key for placement interviews. :contentReference[oaicite:0]{index=0}
Operating systems protect I O devices and device data by controlling access via device files (in Unix) or driver interfaces, ensuring user processes cannot perform illegal I O instructions directly. This is an important security and resource management concept.
A livelock is a situation where processes or threads continuously change their state in response to one another but none makes progress. Unlike deadlock (complete standstill because of waiting), livelock is active but unproductive. For example, two processes repeatedly yield to each other’s resource requests but neither gets the resource. In interviews mentioning contrast with deadlock, detection difficulty and prevention strategies adds depth.
The operating system acts as the intermediary between hardware and application software. It manages resources like CPU, memory, and I O so applications can run efficiently and correctly.
A process is an independent execution unit with its own address space and resources. A thread is a lightweight unit within a process that shares that address space and resources but has its own stack and program counter. This enables threads to communicate more easily and switch faster, but also means we must handle synchronization carefully when using threads.