Modern general-purpose computers, including personal computers and mainframes, have an
operating system to run other programs, such as application software. Examples of operating
systems for personal computers include Microsoft Windows, Mac OS (and Darwin), Unix, and
The lowest level of any operating system is its kernel. This is the first layer of software
loaded into memory when a system boots or starts up. The kernel provides access to various
common core services to all other system and application programs. These services include, but are
not limited to: disk access, memory management, task scheduling, and access to other hardware
As well as the kernel, an operating system is often distributed with tools for programs to
display and manage a graphical user interface (although Windows and the Macintosh have these
tools built into the operating system), as well as utility programs for tasks such as managing files
and configuring the operating system. They are also often distributed with application software that
does not relate directly to the operating system's core function, but which the operating system
distributor finds advantageous to supply with the operating system.
The delineation between the operating system and application software is not precise, and is
occasionally subject to controversy. From commercial or legal points of view, the delineation can
depend on the contexts of the interests involved. For example, one of the key questions in the
United States v. Microsoft antitrust trial was whether Microsoft's web browser was part of its
operating system, or whether it was a separable piece of application software.
Like the term "operating system" itself, the question of what exactly should form the
"kernel" is subject to some controversy, with debates over whether things like file systems should
be included in the kernel. Various camps advocate microkernels, monolithic kernels, and so on.
Operating systems are used on most, but not all, computer systems. The simplest computers,
including the smallest embedded systems and many of the first computers did not have operating
systems. Instead, they relied on the application programs to manage the minimal hardware
themselves, perhaps with the aid of libraries developed for the purpose. Commercially-supplied
operating systems are present on virtually all modern devices described as computers, from personal
computers to mainframes, as well as mobile computers such as PDAs and mobile phones.