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OS · Question Set
I/O Systems & Device Management interview questions for placements and exams.
Questions
11
Included in this set
Subject
OS
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Difficulty
Mixed
Level of this set
Go through each question and its explanation. Use this set as a focused practice pack for OS.
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.
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Spooling stands for Simultaneous Peripheral Operations On-Line. It uses an intermediate buffer or queue so that data destined to a slower device is stored temporarily and the system can move on. This improves efficiency for devices like printers.
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.
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 device driver is software (often part of the OS kernel or loaded module) that abstracts hardware specifics and provides a uniform interface for higher layers of the OS. It handles initialization, data transfer, interrupts and cleanup for a device. This is often asked in interviews to test understanding of I O subsystems.
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.
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 cache is a small fast memory that stores copies of frequently accessed data or instructions to reduce access latency, often between CPU and main memory. A buffer is a temporary storage area used when data is moving between two devices or between device and memory, often to accommodate speed mismatch. The key difference is purpose: caching aims for reducing access time by exploiting locality, buffering aims for smoothing data transfer. In I O systems you might buffer disk writes and also cache disk reads; distinguishing these roles and trade-offs shows thorough understanding in interviews.
Programmed I O is the simplest I O method where the CPU repeatedly checks device status and then transfers data, causing the CPU to be busy with I O. Interrupt-driven I O improves efficiency: the device signals the CPU when it is ready and the CPU handles the transfer only then. Direct Memory Access (DMA) is the most efficient: the device controller or DMA hardware moves data between memory and device without busy CPU, freeing the CPU for other tasks. The trade-off is added complexity in hardware and handling synchronization of memory and bus access.
Direct Memory Access (DMA) permits devices to transfer data to or from main memory directly without continuous CPU intervention. A dedicated DMA controller requests control of the system bus, transfers a block of data, and then returns control to the CPU. The major advantages are: the CPU is freed from busy waiting for I O, overall system efficiency increases, large data transfers are more efficient, and I O throughput improves. The trade-off is increased complexity in the I O subsystem, handling of bus arbitration and ensuring coherence of data between CPU caches and memory.
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.