Operating System Kernel and User Mode
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Questions and Answers

Which of the following errors might be recoverable by a program?

  • System crash
  • Bus error (correct)
  • Segmentation fault (correct)
  • Divide by zero (correct)

    • What typically occurs as a result of an abort?

  • Recovery of processes in user mode
  • The program continues running normally
  • The kernel handles the error silently
  • Rebooting or shutting down the computer (correct)

    • What describes a trap in systems programming?

  • An error that leads to a program malfunction
  • A software-generated exception handled in user mode
  • A mechanism to change protection domains safely (correct)
  • A hardware-generated error requiring immediate attention

    • Which of the following best defines an abort?

    <p>An unrecoverable error resulting in terminating processes</p> Signup and view all the answers

    What is a key feature of trap handlers?

    <p>They allow programs to call kernel functions in supervisor mode</p> Signup and view all the answers

    What type of exception is caused by hardware signaling the operating system?

    <p>Interrupts</p> Signup and view all the answers

    Which exception type is specifically mentioned as being potentially recoverable by the kernel?

    <p>Faults</p> Signup and view all the answers

    What is the main focus regarding exceptions in the content provided?

    <p>Traps</p> Signup and view all the answers

    Which type of exception is due to unrecoverable errors outside of software control?

    <p>Aborts</p> Signup and view all the answers

    Which of the following is an example of a synchronous exception?

    <p>Segmentation fault</p> Signup and view all the answers

    What action may the kernel take when a fault occurs?

    <p>Terminate the program or shut down</p> Signup and view all the answers

    What causes traps as mentioned in the content?

    <p>Software initiated exceptions</p> Signup and view all the answers

    Which of the following is not a type of exception listed?

    <p>Exceptions</p> Signup and view all the answers

    What is the primary role of the operating system kernel?

    <p>To manage hardware and provide protection against misbehaving programs.</p> Signup and view all the answers

    In which mode do user programs operate?

    <p>User mode.</p> Signup and view all the answers

    What characterizes exceptions in control flow?

    <p>They allow non-local transfer of control.</p> Signup and view all the answers

    How does a system call function within a program?

    <p>It facilitates synchronous but non-local transfer of control.</p> Signup and view all the answers

    What must user mode programs do to access shared resources?

    <p>Contact the kernel for permission to access resources.</p> Signup and view all the answers

    What are the types of events that can cause exceptions?

    <p>Both hardware and software triggers.</p> Signup and view all the answers

    Which statement about the operating system and kernel is incorrect?

    <p>The operating system only includes user applications.</p> Signup and view all the answers

    What is the consequence of a user mode program misbehaving?

    <p>The kernel will intervene to prevent system instability.</p> Signup and view all the answers

    What is the main purpose of a system call in a user-mode program?

    <p>To invoke kernel functions</p> Signup and view all the answers

    What happens when a system call instruction is executed?

    <p>The system enters supervisor mode</p> Signup and view all the answers

    Which of the following is NOT a step taken by the kernel when it receives a system call?

    <p>Executing arbitrary user code</p> Signup and view all the answers

    What is an example of a dedicated hardware instruction for system calls?

    <p>syscall</p> Signup and view all the answers

    Why is it important that the user mode cannot control what code the kernel runs?

    <p>To prevent malicious or buggy programs from affecting the system</p> Signup and view all the answers

    Which of the following is a correct statement regarding system calls?

    <p>The C library makes system calls appear like regular C functions.</p> Signup and view all the answers

    What is the primary consequence of executing a system call in terms of performance?

    <p>It introduces a substantial overhead.</p> Signup and view all the answers

    Which of the following functions is classified as a system call?

    <p>open()</p> Signup and view all the answers

    What is one reason programs should minimize system calls?

    <p>They take tens to hundreds of thousands of clock cycles.</p> Signup and view all the answers

    Which statement about protection domains is correct?

    <p>They control access to hardware resources.</p> Signup and view all the answers

    What role do exception handlers play in the system?

    <p>They run in supervisor mode in the kernel.</p> Signup and view all the answers

    Which of these statements is true regarding special trap exceptions?

    <p>They are used to implement system calls.</p> Signup and view all the answers

    Why is it beneficial for user mode programs to request access to the kernel?

    <p>It provides increased system efficiency.</p> Signup and view all the answers

    What effect do cache effects have on system calls?

    <p>They can increase the time taken for system calls.</p> Signup and view all the answers

    What action should programs ideally take regarding system calls?

    <p>Minimize system calls when practical.</p> Signup and view all the answers

    What is the primary function of system calls in a program?

    <p>To allow user mode programs to access kernel services.</p> Signup and view all the answers

    What is the primary function of the kernel within a computer system?

    <p>To access hardware directly and manage system resources</p> Signup and view all the answers

    Which of the following statements about protection domains is true?

    <p>Protection domains are a hardware capability that restricts access.</p> Signup and view all the answers

    What happens when an exception occurs in a user mode program?

    <p>Control passes to the kernel to handle the exception.</p> Signup and view all the answers

    What is a characteristic of changing protection domains?

    <p>It is a supervisor mode operation that can be slow and expensive.</p> Signup and view all the answers

    Which option describes the relationship between user mode and supervisor mode?

    <p>User mode has less access than supervisor mode, which has full access to the hardware.</p> Signup and view all the answers

    Why is changing protection domains considered a controlled operation?

    <p>Supervision ensures only authorized changes are made.</p> Signup and view all the answers

    Which of the following is NOT a function of the kernel in supervisor mode?

    <p>Run user applications</p> Signup and view all the answers

    What type of protection does the hardware provide in user mode?

    <p>Strong access restrictions to prevent unauthorized actions</p> Signup and view all the answers

    Study Notes

    Kernel and User Mode

    • The operating system, or kernel, manages hardware.
    • It supports a dedicated computer model.
    • It protects against misbehaving programs.
    • The kernel is the core of the operating system.
    • In essence, the operating system and kernel are just programs.

    User Mode Programs

    • Programs run in user mode.
    • User mode programs act as if they have their own dedicated computer.
    • User mode programs need the kernel to access shared resources.

    Exceptions

    • Exceptions are a type of control flow different from if-then-else statements.
    • Exceptions allow a program to transfer control to another function or program.
    • They can be triggered by external events (asynchronous).
    • Exceptions are caused by hardware or software.
    • Exception handling requires both hardware and software components.

    System Calls

    • A system call is a specific type of exception.
    • It lets a program leave its dedicated computer and connect with the kernel.
    • System calls are synchronous but non-local, meaning they change control without being dependent on any other external state.

    The Kernel and Supervisor Mode

    • The kernel doesn't have a dedicated computer.
    • The kernel controls the "real computer".
    • It operates in a special mode, often called supervisor mode.
    • The kernel has direct access to hardware.
    • It manages virtual memory mappings and process memory.

    Protection Domains

    • Protection domains represent the allowed access level for a process to the computer's hardware resources.
    • Supervisor mode is a higher-privileged protection domain.
    • User mode is a lower-privileged protection domain.
    • Protection domains are a hardware feature.
    • User programs run in user mode; the kernel runs in supervisor mode.
    • Hardware enforces access restrictions on user mode.
    • Some hardware has more than two protection domains.

    Changing Protection Domains

    • Changing protection domains is a supervisor mode operation.
    • This action safeguards against programs leaving user mode.
    • Switching domains requires a well-defined method, like exceptions.
    • Changing protection domains can be computationally expensive.

    Exception Flow

    • When an exception happens, the control shifts to the kernel.
    • If control is already with the kernel, its location changes.
    • If the control is within a user mode program, contexts switch and the kernel takes over.

    Types of Exceptions

    • Interrupts are asynchronous signals from hardware.
    • Traps are synchronous exceptions caused by software (on purpose).
    • Faults are synchronous exceptions from software due to potentially recoverable errors.
    • Aborts are synchronous exceptions from irreparable errors outside of software control.

    Interrupts

    • Hardware uses interrupts to signal the operating system.
    • Examples include network packets arriving, clocks ticking, or disks finishing reading.
    • Interrupts are handled by the kernel.

    Faults

    • Faults are recoverable program errors.
    • When a fault occurs, the kernel can try to fix it or notify the program.
    • The kernel might choose to terminate the program or shut down.
    • Examples of faults include segmentation faults and divide-by-zero errors.

    Fault Recovery

    • Some faults are not errors; for example, page faults deal with bringing in new pages.
    • Other faults might be recoverable by the program.
    • Such faults include divide by zero, segmentation faults, and bus errors.
    • An error doesn't always have to be fatal (e.g., a potentially usefully value can be returned instead).

    Aborts

    • Aborts represent unrecoverable errors, frequently resulting in computer rebooting or shutting down.
    • The kernel manages abort handling.

    Traps

    • Traps are software-generated exceptions.
    • They are created by special instructions within a program.
    • A key feature of traps is that they are handled by the kernel in supervisor mode.
    • This enables user-mode programs to call into the kernel safely, facilitating protection domain changes.

    System Calls

    • System calls are traps used by user-mode programs to interact with kernel functions.
    • Many operating systems have specific hardware instructions for system calls (e.g., ARM's SVC, x86-64’s syscall).

    System Call Handling

    • The hardware switches to supervisor mode when a system call instruction executes.
    • It then invokes a specific kernel routine.
    • Kernel actions include identifying program requirements, verifying inputs, authenticating requests, and performing operations or indicating failures.
    • The kernel makes decisions for accessing resources outside the program's dedicated environment.

    Implications of Traps

    • User mode cannot control what the kernel does.
    • The hardware dictates and configures kernel operations.
    • Modern operating systems use this to guard themselves from buggy or malicious programs.

    Invoking a System Call

    • System calls, like open(), sbrk(), mmap(), are necessary for interactions outside the program's dedicated environment.
    • The C library makes system calls appear as regular C functions.

    Overhead

    • System calls are slow, taking tens to hundreds of thousands of clock cycles.
    • This slowness is primarily due to changes in protection domains, validation of input data, memory adjustments, and cache effects.

    Summary

    • Exceptions are special control flows.
    • Protection domains control access to hardware resources.
    • Exception handlers run in supervisor mode within the kernel.
    • Special trap exceptions facilitate system calls.
    • System calls allow user-mode programs to get access to the kernel.

    References

    • The provided text lists book titles and authors, along with URLs (these are not summarized, but provided as reference material)

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    Description

    This quiz explores the concepts of kernel and user mode within operating systems. It covers how the kernel manages hardware, the function of user mode programs, and the role of exceptions and system calls. Test your knowledge on these fundamental aspects of operating system architecture.

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