Practice · Hardware Support, System Calls and OS Architecture

Q1

On a 64-bit Linux system, which system call sends a signal to a process?

A execve B kill C fork D open

Q2

A program divides by zero. What kind of event transfers control to the OS?

A A DMA transfer B An exception (fault) raised by the CPU C A hardware interrupt from a device D A system call requested by the program

Q3

What is the role of the interrupt vector table?

A It stores the list of all files. B It stores user passwords. C It maps each interrupt number to the address of its handler. D It lists the processes waiting for the CPU.

Q4

While the disk interrupt handler runs, a higher-priority timer interrupt arrives. In a system with nested interrupts, what happens?

A The timer interrupt is discarded. B The whole system restarts. C The disk handler is paused, the timer handler runs, then the disk handler resumes. D Both handlers run on the same CPU at exactly the same time.

Q5

A 1 GHz CPU polls a device 1,000,000 times per second, and each poll costs 400 cycles. What fraction of the CPU does polling use?

A 40% B 4% C 400% D 0.4%

Q6

For the same 4 MB/s device, the system switches to DMA with one interrupt per 4,096-byte block, costing 1,500 cycles per block (setup + interrupt). About how much CPU does it use on a 1 GHz CPU?

A 50% B 15% C 1.5% D 0.15%

Q7

Why can interrupt-driven I/O be worse than polling for a very fast device?

A With one interrupt per small piece of data, the per-interrupt overhead (saving state, running the handler) is paid extremely often. B Polling never uses the CPU. C Interrupts do not work with fast devices. D Fast devices do not need the OS.

Q8

Which component knows the registers and protocol of a specific network card and offers a uniform interface to the rest of the kernel?

A The boot loader B The shell C The device driver D The interrupt vector table

Q9

Which instruction must be privileged (allowed only in kernel mode)?

A Setting the hardware timer B Adding two registers C Calling a function in the same program D Reading a local variable

Q10

A user program executes an instruction that disables interrupts. What happens?

A The CPU switches to kernel mode and continues the program. B The instruction is silently ignored by the compiler. C Interrupts are disabled for the whole system. D The hardware detects a privileged instruction in user mode, raises an exception, and the OS handles it (usually by terminating the program).

Q11

A process has base register 30000 and limit register 12000. Which address causes a protection exception?

A 35500 B 30000 C 42000 D 41999

Q12

Why does the OS set a timer before giving the CPU to a user program?

A To guarantee that the OS regains control even if the program never gives the CPU back (e.g., an infinite loop) B To speed up the program C To measure the program's memory usage D To encrypt the program

Q13

Timer interrupts occur 1,000 times per second and each costs 2 µs. What fraction of CPU time do they consume?

A 0.02% B 20% C 2% D 0.2%

Q14

In what order do these steps of a Linux system call happen? (1) kernel looks up the number in the system-call table; (2) library places the call number and arguments in registers; (3) syscall instruction traps into kernel mode; (4) result returned and CPU back in user mode.

A 1, 2, 3, 4 B 2, 3, 1, 4 C 3, 2, 1, 4 D 2, 1, 3, 4

Q15

Which Linux system call creates a new process that is a copy of the caller?

A fork B wait4 C kill D execve

Q16

A program writes 1,000,000 bytes one byte per write call. Assuming 1 µs per system call plus 1 ns per byte, about how long does it take? And with a 4,096-byte buffer?

A About 10 ms and 5 ms B About 1,001 ms unbuffered and about 1.2 ms buffered C The same in both cases D 1 ms and 1 ms

Q17

Why does the kernel check the arguments of every system call (for example, that a buffer pointer really lies in the caller's memory)?

A Because user programs are untrusted; without checks a program could make the kernel read or overwrite memory it must not access B Checks are optional and usually skipped C Because the compiler requires it D To make system calls faster

Q18

What is the correct boot order on a modern PC?

A Kernel → firmware → boot loader → PID 1 B Firmware (UEFI/BIOS) → boot loader (e.g., GRUB) → kernel → PID 1 (systemd/init) C PID 1 → kernel → firmware D Boot loader → firmware → PID 1 → kernel

Q19

In which architecture do device drivers and file systems run as user-mode servers that communicate by message passing?

A Serial processing B Microkernel C Monolithic kernel D Batch monitor

Q20

What is the main disadvantage of a monolithic kernel such as Linux?

A It cannot run on servers. B It cannot load modules. C A bug in any kernel component, such as a driver, runs with full privileges and can crash or compromise the whole system. D It is always slower than a microkernel.

Q21

Which OS family is commonly described as a hybrid kernel?

A Windows NT family B Punched-card batch monitors C The THE multiprogramming system D MS-DOS

Q22

A car maker needs an OS for a brake-control unit where a crashed component must never take down the whole system and deadlines are strict. Which choice is most appropriate?

A A batch OS B A real-time microkernel OS such as QNX C A general-purpose monolithic desktop OS D No OS at all, to save time

Q23

What does strace -c ls show?

A The CPU temperature B The source code of ls C The list of users D A summary of the system calls ls made — how many of each and the time spent

Q24

How does a user program legally switch the CPU from user mode to kernel mode?

A By calling any function named kernel B By writing directly to the mode bit C By running as the administrator in the shell D Only through an interrupt, a trap (system call) or an exception, which jump to handlers chosen by the OS

Q25

Why does openat usually return file descriptor 3 for the first file a program opens?

A Because 3 is the size of the file. B Because the kernel reserves 0–2 for the network. C Because file descriptors start at 3 by law. D Because descriptors 0, 1 and 2 are already open for standard input, standard output and standard error.

Q26

What is a unikernel?

A A type of disk B A kernel that supports only one user C A single-purpose image in which one application is compiled together with only the OS components it needs D The first process started at boot

Q27 Short answer

A 2 GHz CPU handles a network card that receives 50,000 packets per second. Compare the CPU cost of (a) one interrupt per packet at 2,000 cycles each and (b) "interrupt coalescing" — one interrupt per 32 packets at 4,000 cycles each. Show the computation and explain the trade-off.

Q28 Short answer

Describe, step by step, what happens from the moment a Java program calls System.out.println("hi") until the text reaches the terminal, naming the modes, the trap and the kernel's checks.

Q29 Short answer

Choose an OS architecture for each EdgeCampus device and justify: (a) the building's GPU edge server running containers; (b) the electronic door-lock controller of the lab; (c) a tiny battery-powered air-quality sensor.

Q30 Short answer

Explain why an operating system cannot provide protection without hardware support. Give three hardware mechanisms and what each prevents.