What are system calls and how do they function?

System calls are interfaces between a process and the operating system that allow the process to request services from the operating system.

In more detail, system calls provide an essential interface between a process and the operating system. They are a type of procedure call made via a software interrupt by an active process, which provides a service that the operating system controls. This service could be anything from creating a new process, to reading and writing files, or communicating with hardware devices.

When a process makes a system call, it triggers a switch from user mode to kernel mode. User mode is a restricted processing mode designed for applications, while kernel mode is a privileged mode designed for operating system code. This switch is necessary because the services provided by system calls often require a higher level of privilege than what is available in user mode.

The process of making a system call involves several steps. First, the process must set up the specific system call number and any arguments it requires. This is typically done by storing them in a specific location, such as a register. The process then triggers a software interrupt, which signals the operating system to switch to kernel mode and execute the system call. Once the system call is complete, control is returned to the process, and the operating system switches back to user mode.

System calls are crucial for the functioning of any operating system. They provide a way for processes to interact with the operating system and hardware in a safe and controlled manner. Without system calls, processes would have to directly access hardware or memory, which could lead to errors and system instability. By providing a controlled interface, system calls help to maintain the stability and security of the system.

In summary, system calls are a fundamental part of how processes interact with the operating system. They provide a safe and controlled interface for processes to request services from the operating system, helping to maintain system stability and security.