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. This may include hardware-related services (for example, accessing a hard disk drive), creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system

1. Operating System 11
System Calls
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

2. What is a System Call
• 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.
• This may include hardware-related services (for
example, accessing a hard disk drive), creation
and execution of new processes, and
communication with integral kernel services such
as process scheduling.
• System calls provide an essential interface
between a process and the operating system.

3. System Calls Originate from
User Processes
• In most systems, system calls are possible to
be made only from userspace processes.
• In some systems, OS/360 and successors for
example, privileged system code also issues
system calls.

4. System Calls ensure Safety and
Security
• The architecture of most modern processors, with the
exception of some embedded systems, involves a security
model.
• For example, the rings model or layered approach specifies
multiple privilege levels under which software may be
executed: a program is usually limited to its own address
space so that it cannot access or modify other running
programs or the operating system itself, and is usually
prevented from directly manipulating hardware devices
(e.g. the frame buffer or network devices).
• However, many normal applications obviously need access
to these components, so system calls are made available by
the operating system to provide well defined, safe
implementations for such operations.

5. Interrupts
• The operating system executes at the highest level of
privilege, and allows applications to request services
via system calls, which are often initiated via interrupts.
• An interrupt automatically puts the CPU into some
elevated privilege level, and then passes control to the
kernel, which determines whether the calling program
should be granted the requested service.
• If the service is granted, the kernel executes a specific
set of instructions over which the calling program has
no direct control, returns the privilege level to that of
the calling program, and then returns control to the
calling program.

6. System Calls
• Programming interface to the services provided by the
OS
• Typically written in a high-level language (C or C++)
• Mostly accessed by programs via a high-level Application
Programming Interface (API) rather than direct system
call use
• Three most common APIs are Win32 API for Windows,
POSIX API for POSIX-based systems (including virtually all
versions of UNIX, Linux, and Mac OS X), and Java API for
the Java virtual machine (JVM)
Note that the system-call names used throughout this
text are generic

7. Example of System Calls
• System call sequence to copy the contents of
one file to another file

8. System Call Implementation
• Typically, a number associated with each system call
– System-call interface maintains a table indexed
according to these numbers
• The system call interface invokes the intended
system call in OS kernel and returns status of the
system call and any return values
• The caller need know nothing about how the
system call is implemented
– Just needs to obey API and understand what OS will do
as a result call
– Most details of OS interface hidden from programmer
by API
• Managed by run-time support library (set of functions built
into libraries included with compiler)

9. API – System Call – OS Relationship

10. System Call Parameter Passing
• Often, more information is required than simply
identity of desired system call
– Exact type and amount of information vary according to
OS and call
• Three general methods used to pass parameters to
the OS
– Simplest: pass the parameters in registers
• In some cases, may be more parameters than registers
– Parameters stored in a block, or table, in memory, and
address of block passed as a parameter in a register
• This approach taken by Linux and Solaris
– Parameters placed, or pushed, onto the stack by the
program and popped off the stack by the operating
system
– Block and stack methods do not limit the number or
length of parameters being passed

11. Parameter Passing via Table

12. System Call Implementation
• In a multithreaded process system calls can be
made from multiple threads.
• The handling of such calls is entirely dependent
on the design of the specific operating system.
• The following slides shows typical models
followed by kernels
• Many-to-one model
• One-to-one model
• Hybrid model

13. Many-to-one model:
• All system calls from any user thread in a
process are handled by a single kernel-level
thread.
• This model has a serious drawback – any
blocking system call (like awaiting input from
user) can freeze all the other threads.
• Also, since only one thread can access the
kernel at a time, this model cannot utilize
multiple cores of processor.

14. One-to-one model
• Every user thread gets attached to a distinct
kernel-level thread during a system call.
• This model solves the above problem of
blocking system calls.
• It is found in all major distribution of Linux,
recent Windows and Solaris versions.

15. Many-to-many model
• In this model a pool of user threads is mapped
to a pool of kernel threads. All system calls
from a user thread pool are handled by the
threads in their corresponding kernel thread
pool

16. both many to many and one to one
model depending upon choice made by
the kernel. This is found in old
versions of IRIX, HP-UX and Solaris.

17. System Calls as interface
between Running Programming
and OS
• System calls provide the interface between a running
program and the operating system.
• Generally available as assembly-language instructions.
• Languages defined to replace assembly language for
systems programming allow system calls to be made
directly (e.g., C, C++)
• Three general methods are used to pass parameters between
a running program and the operating system.
• Pass parameters in registers.
• Store the parameters in a table in memory, and the table
address is passed as a parameter in a register.
• Push (store) the parameters onto the stack by the program,
and pop off the stack by operating system.

18. Types of System Calls
• Process control
• File management
• Device management
• Information maintenance
• Communications

19. Process control
• end, abort
• load, execute
• create process, terminate process
• get process attributes, set process attributes
• wait for time
• wait event, signal event
• allocate and free memory

20. File manipulation
• create file, delete file
• open, close
• read, write, reposition
• get file attributes, set file attributes

21. Device manipulation
• request device, release device
• read, write, reposition
• get device attributes, set device attributes
• logically attach or detach devices

22. Information maintenance
• get time or date, set time or date
• get system data, set system data
• get process, file, or device attributes
• set process, file, or device attributes

23. Communications
• create, delete communication connection
• send, receive messages
• transfer status information
• attach or detach remote devices

24. Assignment
• Explain the concept of System Call