In computing

Computing

Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, a system call is how a program requests a service from an operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

's kernel

Kernel (computing)

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...

. This may include hardware related services (for e.g. accessing the Hard Disk), creating and executing new processes, and communicating with integral kernel services(like scheduling). System calls provide the interface between a process and the operating system.
The design of the microprocessor architecture on practically all modern systems (except some embedded systems) involves a security model

CPU modes

CPU modes are operating modes for the central processing unit of some computer architectures that place restrictions on the type and scope of operations that can be performed by certain processes being run by the CPU...

 (such as the rings

Ring (computer security)

In computer science, hierarchical protection domains, often called protection rings, are a mechanism to protect data and functionality from faults and malicious behaviour . This approach is diametrically opposite to that of capability-based security.Computer operating systems provide different...

model) which specifies multiple privilege levels under which software may be executed; for instance, a program is usually limited to its own address space

Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...

 so that it cannot access or modify other running programs or the operating system itself, and a program is usually prevented from directly manipulating hardware devices (e.g.