Operating Systems Overview

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Questions and Answers

What are the two main functions of an operating system?

  • Providing security and managing hardware components
  • Managing user programs and providing abstractions
  • Handling system calls and managing user accounts
  • Providing abstractions and managing computer resources (correct)

Why is the resource management function of an operating system largely transparent to users?

  • Because the operations are performed automatically and without user intervention. (correct)
  • Because users generally do not have the technical expertise to manage resources.
  • Because resource management is handled entirely by hardware components.
  • Because modern operating systems are designed to be user-friendly.

Which of these is NOT an example of an abstraction provided by an operating system?

  • Reading data from a file
  • Writing data to a file
  • Managing the physical memory of the system (correct)
  • Creating a file

Why is it necessary to understand the system call interface when studying operating systems?

<p>It helps in understanding how the operating system interacts with user programs. (B)</p> Signup and view all the answers

What is the main challenge faced when discussing system calls?

<p>The variation in system calls across different operating systems. (B)</p> Signup and view all the answers

What is the primary function of the read system call in the context provided?

<p>To read data from a specified file (B)</p> Signup and view all the answers

What is the primary purpose of a procedure library in the context of system calls?

<p>To facilitate making system calls from high-level programming languages. (C)</p> Signup and view all the answers

Which of the following parameters is passed by reference in the read system call?

<p>Buffer (buffer) (B)</p> Signup and view all the answers

How does a process switch from user mode to kernel mode when requesting a system service?

<p>By executing a special system call instruction. (C)</p> Signup and view all the answers

What is the key difference between a system call and a procedure call?

<p>System calls involve interaction with the operating system kernel, while procedure calls do not. (B)</p> Signup and view all the answers

What does the return value of the read system call represent?

<p>The number of bytes actually read from the file (D)</p> Signup and view all the answers

In the context of the content provided, what does the term 'end-of-file' refer to?

<p>The point at which the file has been completely read (C)</p> Signup and view all the answers

What is the primary purpose of the TRAP instruction in the read system call process?

<p>To switch from user mode to kernel mode (B)</p> Signup and view all the answers

Why are parameters pushed onto the stack in reverse order when making a read system call?

<p>To accommodate historical reasons related to the printf function (C)</p> Signup and view all the answers

What is the significance of the global variable 'errno' in the context of system calls?

<p>It indicates the reason for a system call failure (B)</p> Signup and view all the answers

What type of instruction is used to transfer control from the user program to the kernel during the read system call?

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

What is the primary difference between a TRAP instruction and a procedure call instruction?

<p>TRAP instructions are executed by the operating system, while procedure calls are executed by the user program. (A), TRAP instructions change the mode of the system to kernel mode, while procedure calls do not. (G), TRAP instructions are used for system calls, while procedure calls are used for regular function calls. (H)</p> Signup and view all the answers

What is the purpose of the system call number in the context of a system call?

<p>It identifies the specific system call to be executed. (D)</p> Signup and view all the answers

How is control returned to the user program after a system call is completed?

<p>The system call handler returns control to the user-space library procedure, which then returns to the user program. (G)</p> Signup and view all the answers

What is the purpose of the stack in the context of system calls?

<p>It stores the return address to the user program. (C), It stores the user program's input parameters to the system call. (D)</p> Signup and view all the answers

What happens if a system call blocks the caller?

<p>The operating system will look for another process to run while the caller is blocked. (D)</p> Signup and view all the answers

What is the primary function of the user-space library procedure in the context of system calls?

<p>To provide an interface between the user program and the kernel. (A)</p> Signup and view all the answers

Which of the following is NOT a category mentioned in the text for the POSIX system calls?

<p>Error Handling (A)</p> Signup and view all the answers

How is the stack pointer updated after a system call returns?

<p>The stack pointer is updated by the user program to remove the parameters that were pushed onto the stack before the system call. (A)</p> Signup and view all the answers

Flashcards

Operating System Functions

Operating systems provide abstractions to user programs and manage computer resources.

User Programs Interaction

Interaction between user programs and the OS mainly involves file operations like creating, reading, and deleting files.

System Calls

System calls are the interface between user programs and the operating system, allowing access to OS features.

Example System Call

An example of a system call is 'read,' which typically includes parameters for file, buffer location, and byte count.

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POSIX Standards

POSIX refers to a set of standards for maintaining compatibility between operating systems, particularly UNIX-like systems.

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Single-CPU Execution

A single-CPU computer can only execute one instruction at a time, impacting multitasking.

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Trap Instruction

A trap instruction is used by a user program to request a system service from the operating system.

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Kernel vs Procedure Calls

System calls enter the kernel for services, while procedure calls do not leave the user program’s context.

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read Function

A library procedure that invokes the system call to read data from a file.

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Parameters of read

Three data points: file descriptor, buffer, and number of bytes to read.

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Count Variable

Stores the number of bytes actually read by the read call.

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End-of-file

A condition indicating no more data can be read from a file.

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Global variable errno

Stores error codes when a system call fails.

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Parameter Passing

First and third parameters are passed by value, while the second is passed by reference.

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Kernel mode

A privileged mode of operation for executing low-level system tasks in the operating system.

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System-call handler

A piece of code that processes a specific system call and executes the requested task.

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Stack pointer

A register that points to the current top of the stack in memory, managing function calls and local variables.

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Blocking call

A system call that prevents a process from continuing until the operation can complete, such as waiting for input.

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User-space library procedure

Functions in user space that facilitate system calls, enabling program interaction with the OS.

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Stack clean-up

The process of removing parameters and resetting the stack after a function call is completed.

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POSIX system calls

Standardized calls in operating systems for performing basic tasks; around 100 in total.

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Study Notes

Operating System Functions

  • Operating systems provide abstractions for user programs and manage computer resources.
  • Resource management is largely transparent to users.
  • The interface between user programs and the operating system deals with abstractions.

System Calls

  • System calls vary between operating systems.
  • Concepts are similar across operating systems, though details differ.
  • Examples include functions for reading, writing, creating, and deleting files.

System Call Mechanics

  • A single CPU can only execute one instruction at a time.
  • If a user program needs a system service (e.g., reading a file), a trap instruction transfers control to the operating system.
  • The operating system determines the process's request from parameters.
  • It executes the system call and returns control to the instruction following the system call.
  • System calls are "special" procedure calls that enter the kernel, unlike normal procedure calls.

Example: read System Call

  • The read system call has three parameters: file descriptor, buffer address, and number of bytes to read.
  • The call takes place using a library procedure with the same name.
  • The number of bytes read is returned via a variable.
  • Error handling is important: checks for invalid parameters or disk errors.

System Call Steps

  • Parameters are pushed onto the stack in reverse order.
  • The library procedure places the system call number in an expected register.
  • A trap instruction switches to kernel mode and executes a system call handler.
  • The system call handler finds the correct handler based on system call number and executes the respective handler.
  • The control is transferred back, and stack cleanup occurs.
  • System calls may or may not block the caller.

POSIX Procedures

  • Various POSIX system calls exist for tasks like file operations, process management, and input/output.
  • The mapping between POSIX calls and system calls is not always one-to-one.
  • Some POSIX procedures can be performed entirely in user space to enhance performance.

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