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Environment in which a piece of software is executed
A platform can be seen both as a constraint on the software development process, in that different platforms provide different functionality and restrictions; and as an assistant to the development process, in that they provide low-level functionality ready-made. For example, an OS may be a platform that abstracts the underlying differences in hardware and provides a generic command for saving files or accessing the network.
Platforms may also include:
Hardware alone, in the case of small embedded systems. Embedded systems can access hardware directly, without an OS; this is referred to as running on "bare metal".
A browser in the case of web-based software. The browser itself runs on a hardware+OS platform, but this is not relevant to software running within the browser.
An application, such as a spreadsheet or word processor, which hosts software written in an application-specific scripting language, such as an Excel macro. This can be extended to writing fully-fledged applications with the Microsoft Office suite as a platform.
Cloud computing and Platform as a Service. Extending the idea of a software framework, these allow application developers to build software out of components that are hosted not by the developer, but by the provider, with internet communication linking them together. The social networking sites Twitter and Facebook are also considered development platforms.
A virtualized version of a complete system, including virtualized hardware, OS, software, and storage. These allow, for instance, a typical Windows program to run on what is physically a Mac.
Some architectures have multiple layers, with each layer acting as a platform to the one above it. In general, a component only has to be adapted to the layer immediately beneath it. For instance, a Java program has to be written to use the Java virtual machine (JVM) and associated libraries as a platform but does not have to be adapted to run for the Windows, Linux or Macintosh OS platforms. However, the JVM, the layer beneath the application, does have to be built separately for each OS.