The operating system (OS) allows users to perform the basic functions of a computer. The OS manages all software and peripheral hardware, and accesses the central processing unit (CPU) for memory or storage purposes. It also makes it possible for a system to simultaneously run applications. All PCs, laptops, tablets, smartphones, and servers require an OS. Developers may use specific operating systems that are more conducive to programming and application development, while the average employee will likely use a proprietary system for more common, everyday usage. The most common operating systems are Microsoft's Windows, Apple's OS X, Linux distributions, and mobile operating systems for smartphones.