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Infosys · OS
Practice OS questions specifically asked in Infosys interviews – ideal for online test preparation, technical rounds and final HR discussions.
Questions
26
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Company
Infosys
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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 Infosys OS round.
Classic IPC mechanisms include shared memory, message queues, pipes, sockets and semaphores. Virtual memory is a memory management technique, not an inter‐process communication channel.
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A system call is the mechanism by which a user program requests a service from the kernel of the operating system. It allows safe transition from user mode to kernel mode to execute privileged operations like file I O, process control or creating threads. It is important because it enforces protection, isolates user code from kernel code, and enables resource management.
A PCB is a data structure used by the OS to keep track of each process. It stores process identifier (PID), process state, program counter, register values, memory limits, list of open files, scheduling information and more. The OS uses the PCB to resume and manage processes.
In priority scheduling, if processes with low priority keep arriving, then a low priority process might never get CPU time. Without an aging mechanism (which increases priority over time), starvation becomes possible. FCFS and non-preemptive SJF will eventually serve everyone; Round Robin gives each process a time slice so starvation is less likely.
Segmentation divides a process’s memory into logical segments such as code, data, stack or other modules. Each segment may be of variable size and can grow or shrink independently. The key difference from paging is that segments are logical units meaningful to the program, while pages are fixed size and meaningless to the program’s logic. Segmentation can suffer from external fragmentation because segments are variable sized. On the other hand paging reduces external fragmentation but may suffer from internal fragmentation, and segments require more complex management. :contentReference[oaicite:3]{index=3}
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.
Free-space management is how a file system tracks which blocks are free for allocation. Two common techniques are a free-list (linked list of free blocks) and a bitmap (array of bits where each bit indicates free or used). The free-list is simple and efficient for sequential allocation but may fragment and traverse slowly for large free lists. A bitmap allows quick access and compact representation, and supports efficient block allocation decisions, but requires scanning and may incur overhead for very large disks. Highlighting trade-offs is important in interview responses.
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}
The SCAN algorithm (sometimes called the elevator algorithm) moves the disk arm in one direction, servicing all requests until it reaches the end, then reverses. This reduces back-and-forth movement and provides better uniform wait times. It is widely referenced in I O scheduling interview questions.
A Trusted Execution Environment (TEE) is a hardware-supported secure area of a processor that ensures confidentiality and integrity of code and data inside it, shielding them from even higher privilege software like the OS kernel. TEEs represent modern OS security extensions and show up in advanced interview discussion.
While both buffering and spooling use temporary storage, buffering typically refers to holding data to smooth or decouple data flow between two entities, and spooling refers to queuing jobs or data for a slower device so that the main system can continue working. This nuanced difference is often discussed in deeper I O questions.
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.
A correct solution to the critical-section problem must satisfy mutual exclusion (only one process at a time), progress (decide which process enters if none is in critical section) and bounded waiting (each process must wait only a bounded number of times). Infinite waiting (starvation) is not acceptable. This is a standard concurrency property.
In a resource allocation graph where each resource has a single instance, a directed cycle implies that a set of processes are waiting in a circular chain for resources held by each other — this indicates a deadlock condition. Recognising graph-based detection is a common interview point.
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.
The SCAN algorithm (also called the elevator algorithm) moves the disk arm in one direction, servicing requests as it goes, until it hits one end of the disk, then reverses direction. This approach reduces arm movement and can offer better average wait times than simple methods. In interview settings, explaining the ‘elevator’ metaphor and its trade-offs (for example with SSTF) is valuable.
Polling (or programmed I/O) is when the CPU repeatedly checks (in a loop) a device status to see if it is ready, then performs the transfer. It wastes CPU cycles while waiting. Interrupt-driven I/O is when the device issues an interrupt when ready, and the CPU handles the request via ISR (Interrupt Service Routine). Interrupts reduce wasteful CPU polling but add overhead due to context switching, interrupt latency and complexity. Polling may be suitable for very fast or predictable devices. Interrupts are better for devices with unpredictable latency or that allow CPU to work on other tasks. In interviews, discuss trade-offs: timing, latency, complexity, context switch overhead, CPU utilisation.
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}
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}
Thrashing occurs when the system spends the majority of time swapping pages in and out of memory instead of executing actual processes. It happens when too many processes are active and there are insufficient frames to hold their working sets. As a result the CPU utilisation drops. Prevention techniques include reducing the degree of multiprogramming, using working set model to determine how many pages a process needs, allocating enough frames, or implementing local page replacement rather than global. Recognising thrashing and advocating prevention shows maturity in interview responses. :contentReference[oaicite:7]{index=7}
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}
Buffering is the technique of using a temporary memory area (buffer) to hold data while it is transferred between two devices or between device and memory. It helps decouple producer and consumer speeds (for example when a fast CPU writes to a slower peripheral). Buffering improves throughput, smooths bursts of data and reduces I/O invocation overhead. On the flip side, buffering consumes memory, can introduce latency (data waits in buffer), may cause inconsistency if not flushed correctly, and adds complexity (flush logic, overflow, memory management). Interviewers like it when you discuss both the benefit and trade-offs.
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.