How are system calls connected to the operating system?

System calls are a method to program for communicating through an operating system. Application developers unable to possess straight access with system calls might be able to access with API (Which indicates the way that software components must communicate).

System call provides an interface between the user program and the operating system. When the user wants to give an instruction to the OS then it will do it through system calls. Or a user program can access the kernel which is a part of the OS through system calls.

It is a programmatic way in which a computer program requests a service from the kernel of the operating system.

The program executes in two modes, which are as follows −

  • User mode − Cannot access any hardware resources, which perform only the user operations.

  • Kernel-mode − Can access hardware resources like RAM, Printer.

The processor in a computer switches between the two modes depending upon what types of code are running on the processor. A process running in the user mode cannot access the virtual addresses that are reserved for the operating system.

The system is in user mode when the operating system is running a user application such as handling a text editor. The transition from the user mode to kernel mode occurs, when the application requests the help of the operating system or an interrupt or a system call occurs.

The mode bit is set to 1 in the user mode. When a program needs any hardware resources, it needs to make a call to the kernel.

The functions which are in the API invoke the normal system calls. System call permits the user-level activity for appealing the services of operating systems.

Program in user mode needs to access the RAM, Program should request the kernel for offering access to RAM which was performed through a system call. The switching of mode from the user to the kernel is known as a context switch.

Note − Kernel mode is strong mode and special. Any occurrence of crashes leads to a complete system that halts which is not observed in user mode where just a specific program halts.

The figure given below depicts the kernel mode −

Updated on: 01-Dec-2021


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