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A virtual machine (VM) is a software implementation of a machine that executes programs like a physical machine. Virtual machines are separated into two major classifications:

1. A system virtual machine provides a complete system platform which supports the execution of a complete operating system (OS).

2. A process virtual machine is designed to run a single program, which means that it supports a single process.

An essential characteristic of a virtual machine is that the software running inside is limited to the resources and abstractions provided by the virtual machine—it cannot break out of its virtual environment.

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Microsoft Windows

Windows XP Embedded Windows Vista

Windows 8.1

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IOS

iPhone OS 1.0

iOS 4

iOS 7

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Android

Android 1.5 Cupcake (API level 3) Android 4.0.1 (Ice Cream Sandwich) Android 4.4 KitKat

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Computer software contrasts with computer hardware, which is the physical component of computers. Computer hardware and software require each other and neither can be realistically used without the other.

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An operating system (OS) is software that manages computer hardware resources and provides common services for computer programs. The operating system is an essential component of the system software in a computer system. Application programs usually require an operating system to function.

The operating system acts for hardware functions as an intermediary between programs and the computer hardware. Almost any device that contains a computer (from cellular phones and video game consoles to supercomputers and web servers) has an operating system.

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The non-tangible component of computers. Executable files,libraries and scripts are computer software. Software consists of clearly defined instructions that upon execution, instructs hardware to perform the tasks for which it is designed.

Examples of software are the following antivirus:

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All the physical, tangible parts of the computer.

The Von Neumann architecture of hardware is composed by a processor, a memory device, an output and input device.

However, nowadays computers have more than output and input device. Furthermore, the processor has a control unit and an arithmetic logic unit.