The operating system.

The underlying hardware in some machines cannot be accessed directly by a program. The operating system (OS) plays the role of mediating access to resources such as memory and I/O facilities. Interactions with operating systems (OS) can take place in the form of a code that causes the execution of a function that is part of the OS. These functions are called system calls.
A personal computer can’t do anything useful unless it’s running an operating system—a basic type of software that acts as a supervisor for all the applications, games, or other programs you use. The operating system sets the rules for using memory, drives, and other parts of the computer. But before a PC can run an operating system, it needs some way to load the operating system from disk to random access memory (RAM). The way to do this is with the bootstrap or simply to boot — a small amount of code that’s permanently a part of the PC.
The 1960’s definition of an operating system is “the software that controls the hardware”. However, today, due to microcode we need a better definition. We see an operating system as the programs that make the hardware useable. In brief, an operating system is the set of programs that controls a computer. Some examples of operating systems are MS-DOS, Windows™, UNIX™, and Linux are the most familiar examples.
Operating Systems are resource managers. The main resource is computer hardware in the form of processors, storage, input/output devices, communication devices, and data. Some of the operating system functions are: implementing the user interface, sharing hardware among users, allowing users to share data among themselves, preventing users from interfering with one another, scheduling resources among users, facilitating