What Is Meant By System Call? Can You Describe Types Of System Calls?

System calls provide an interface between a running program and operating system. System calls are generally available as assembly language instructions. Several higher level languages such as C also allow to make system calls directly.

The system call interface layer contains entry point in kernel code. All system resources are managed by the kernel. Any request from user or application that involves access to any system resource must be handled by kernel code. The user process must not be given open access to kernel code for security reason. Many opening into kernel code called system calls are provided to user so that the user processes can invoke the execution of kernel code. System calls allow processes and users to manipulate system resources.
There are three general methods that are used to pass information (parameters) between a running program and the operating system.

•     One method is to store parameters in registers.
•     Another is to store parameters in a table in memory and pass the address of table.
•     The third method is to push parameters on stack and allow operating system to pop the parameters off the stack.

  Types of System Calls:
The main types of system calls are as follows:
•     Process Control: These types of system calls are used to control the processes. Some examples are end, abort, load, execute, create process, terminate process etc.
•     File Management: These types of system calls are used to manage files. Some examples are Create file, delete file, open, close, read, write etc.
•     Device Management: These types of system calls are used to manage devices. Some examples are Request device, release device, read, write, get device attributes etc.

We can define system call as a function called by an application to invoke a kernel service. The system call checks the arguments given by the application, builds a data structure to convey the arguments to the kernel, and then executes a special instruction called a software interrupt or tap.

System calls work as an interface between a process and the operating system. Normally these calls are embedded in predefined function or subroutine calls. These functions and sub routine may generate a call to a run time routine that makes the system call, or the system call may generated directly in line.

Languages like C, C++ allow system calls to be made directly. UNIX system calls may also be invoked directly from a C, C++ program. System calls occur in different ways, depending on the computer in use. In some cases, more information is required than simply the identity of the desired system call.

A running program can make a request from the operating system at the lowest level. On the other hand a user can issue a request by using a mechanism provided by command interpreter at a higher level. Commands may come from files during batch mod execution or directly from a keyboard when in an inactive or time shared mode. The types of request vary according to the level of the request.
System calls can be grouped into five categories including, Process control, File management, Device management, Information maintenance and communications.

System call provide the interface between a process and the operating system. It is generally available as assemble-language instruction. Several language such as C and C++ have been defined to replace assembly language for system programming. 3 general methods are used to pass parameters between a running program and the operating system:i. Pass parameter in register.ii. Store the parameter in a table in memory, and the table address is passed as a parameter in a register.iii. Push (store) parameter onto the stack by the program, and pop off the stack by operating system.there are 5 types of system calls:1. Process control2. File management3.device management4. Information maintenance5. Communications