System Calls

Summary

This lecture delves into the concept of system calls, explaining their role as an interface to access operating system services. It highlights the crucial distinction between user mode and kernel mode, emphasizing the safety of user mode and the privileged access of kernel mode. The script illustrates the process of system calls through an example of copying file contents, detailing the sequence of system calls required for tasks such as acquiring file names, opening files, reading, writing, and error handling. The lecture concludes by emphasizing the omnipresence of system calls in executing programs and their importance in interfacing with OS services.

Takeaways

• 🔧 System calls provide an interface for programs to access services offered by the operating system.
• 🖥️ Programs can execute in two modes: user mode and kernel mode, with user mode being safer and kernel mode offering direct access to system resources.
• ⚠️ Kernel mode is privileged but risky, as a crash in this mode can bring down the entire system.
• 🔄 Context switching occurs when a program transitions between user mode and kernel mode to access resources.
• 📞 A system call is the mechanism by which a program requests services from the operating system's kernel.
• 📂 In a file copy example, multiple system calls are required to acquire file names, open files, and handle errors.
• 📜 System calls are necessary for tasks like displaying prompts, accepting user input, and accessing hardware devices like keyboards and screens.
• 🔄 Looping system calls are used to read from an input file and write to an output file during the copying process.
• 🚨 Error handling during file operations, such as checking if a file exists, also relies on system calls.
• 💻 Even simple tasks like copying a file involve numerous system calls, highlighting their importance in executing programs on a computer.

Q & A

What are system calls and what purpose do they serve?

System calls provide an interface to the services made available by an operating system. They allow a computer program to request services from the kernel of the operating system, such as accessing resources or performing tasks that require privileged access.

What are the two modes in which a program can execute and what distinguishes them?

The two modes are user mode and kernel mode. In user mode, a program does not have direct access to memory and hardware resources, making it a safer mode of operation. In kernel mode, a program has direct access to system resources, which is a privileged mode but also riskier as a crash can affect the entire system.

Why is kernel mode considered a privileged mode?

Kernel mode is considered privileged because it allows programs to have direct access to memory, hardware, and other system resources, which is not possible in user mode.

What is the consequence of a program crash in kernel mode?

If a program crashes in kernel mode, it can cause the entire system to crash or come to a halt due to its direct access to system resources.

How does a program switch from user mode to kernel mode?

A program switches from user mode to kernel mode through a process known as context switching, which occurs when the program makes a system call to access certain resources or services provided by the operating system.

What is a context switch and when does it occur?

A context switch is the process of switching between user mode and kernel mode. It occurs when a program makes a system call to access system resources, requiring a switch to kernel mode, and also when it switches back to user mode after the call is completed.

What is the role of system calls in the process of copying contents from one file to another?

System calls are used to perform various tasks involved in file copying, such as acquiring file names, opening and creating files, reading from and writing to files, and handling errors or completion messages.

Why is it necessary to use system calls for simple tasks like displaying a prompt on the screen?

System calls are necessary for such tasks because they provide a way for programs in user mode to request services from the kernel, such as accessing hardware output devices like the screen.

How are system calls typically implemented in programming?

System calls are generally available as routines written in C and C++, which can be invoked by programs to request services from the operating system kernel.

What is the significance of system calls in the execution of programs and the operation of an operating system?

System calls are significant as they serve as the primary means for programs to interact with the operating system, allowing them to request services and access resources. They are integral to the functioning of both applications and the operating system itself.

Can you provide an example of a sequence of system calls that might be used in a simple file copying program?

In a simple file copying program, system calls might be used to acquire input and output file names, open the input file, check for existence and create the output file, read from the input file, write to the output file in a loop, handle any errors, close the files, display a completion message, and terminate the program.