This document discusses system calls in operating systems. It defines a system call as the way a program requests a service from the operating system kernel. System calls provide an interface between user programs and the OS and allow processes to access resources. The document describes how system calls work by interrupting a process, executing in kernel mode, then returning control to user mode. It also lists reasons why system calls are needed in operating systems, such as for file I/O, process creation, networking, and hardware device access. Finally, it outlines some common types of system calls like process control, file management, and communications.

1. ADVANCE
OPERATING SYSTEM
Darakhshan Nayab
MSC(IT)
MS(Computer Science)
Dept.of Computer Science

2. Content
■ System Call
■ API
■ How System Call Works?
■ Why do you need System Calls in OS?
■ Types

3. System Call
■ System Call provides an Interface between user program & Operating
system.
■ In computing, a system call is the programmatic way in which a computer
program requests a service from the kernel of the operating system it is
executed on.
■ A system call is a way for programs to interact with the operating system.

4. API
■ Application Program Interface(API) provides an interface
between a process and operating system to allow user-level
processes to request services of the operating system.
System calls are the only entry points into the kernel
system. All programs needing resources must use system
calls.

5. How System Call Works?
Step 1) The processes executed in the user mode till the time a
system call interrupts it.
Step 2) After that, the system call is executed in the kernel-
mode on a priority basis.
Step 3) Once system call execution is over, control returns to the
user mode.,
Step 4) The execution of user processes resumed in Kernel
mode.

7. Why do you need System Calls
in OS?
■ Reading and writing from files demand system calls.
■ If a file system wants to create or delete files, system calls are
required.
■ System calls are used for the creation and management of new
processes.
■ Network connections need system calls for sending and
receiving packets.
■ Access to hardware devices like scanner, printer, need a system
call.

8. Types
■ Process Control
■ File Management
■ Device Management
■ Information Maintenance
■ Communications