System calls allow programs to request services from the operating system kernel. They can be invoked from assembly language or C/C++ and switch the CPU between user and kernel mode. Parameters are typically passed through registers, memory blocks, or the stack. Common system calls manage processes, files, devices, system information, and communication between programs.

1. System Call
 The mechanism used by an application program
requests a service from the kernel of the operating
system.
• These calls are generally available as Assembly
language instruction.
• UNIX System calls can be invoked directly from a C or
C++ program.

3. System call mode
 User Mode
 Kernel Mode
 In kernel mode, the CPU has instructions to manage memory
and how it can be accessed, plus the ability to access
peripheral devices like disks and network cards. The CPU can
also switch itself from one running program to another.
 In user mode, access to memory is limited to only some memory
locations, and access to peripheral devices is denied. The
ability to keep or relinquish the CPU is removed, and the CPU
can be taken away from a program at any time

5. Methods of Passing Parameters to Operating
System
 The simplest is approach is to pass
parameter through registers.
• The second is by storing parameters in blocks and
passing the address through registers
• Finally parameter can be pushed into stack by the
program and popped of by the Operating System.

7. Standard C Library Example
C program invoking printf() library call, which calls write() system call

8. System API Standards
 Three most common API standards are
 POSIX API for POSIX-based systems (including
virtually all versions of UNIX, Linux, and Mac OS X)
 Win32 API for Windows
 Java API for the Java virtual machine (JVM)

9. Categories of System Calls
System calls can be grouped into five major
categories as follows.
• Process control
• File management.
• Device management
• Information Maintenance and
• Communication.

10. Processcontrol
Some system calls under process control are:
 End, abort
 Load, execute
 Create process, terminate process
 Get process, terminate process
 Wait for time
 Allocate and free memory

11. File management.
Some system calls under file management are:
 Create file, delete file
 Open , close
 Read, write, reposition.
 Get file attributes, set fil attributes

12. Device management
Some system calls under device management are:
 Request Device, release device
 Read, write, reposition.
 Get device attributes and set device attributes
 Logically attach or detach devices

13. Information Maintenance
Some system calls under information maintenance are:
 Get time or date, Set time of date
 Logically attach or detach devices
 Information maintenance
 Get system data, Set Systems data
 Get process, file of device attributes
 Set process, file or device attributes

14. Communication
 Create, delete communication connection.
 Send, receive messages
 Transfer status information
 Attach or detach remote devices.