What is it?
John Von Neumann was important scientist and engineer which, among other things, proposed in a high level of abstraction, a general system architecture which is used to this day.

It consists in four elements:
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MemoryStores data, variables, and all instructions of the program. 
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ProcessorActually executes the instructions read from memory into the data also in memory, in cycles of read-execute-write. 
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Input/ OutputRepresents all devices which interact with the bus, as keyboards, mouses, displays. 
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BusThe bus connects the memory, processor and all I/O devices together.