Random-access machine

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In computer science, random-access machine (RAM or RA-machine) is a model of computation that describes an abstract machine in the general class of register machines. The RA-machine is very similar to the counter machine but with the added capability of 'indirect addressing' of its registers. The 'registers' are intuitively equivalent to main memory of a common computer, except for the additional ability of registers to store natural numbers of any size. Like the counter machine, the RA-machine contains the execution instructions in the finite-state portion of the machine (the so-called Harvard architecture).

The RA-machine's equivalent of the universal Turing machine – with its program in the registers as well as its data – is called the random-access stored-program machine or RASP-machine. It is an example of the so-called von Neumann architecture and is closest to the common notion of a computer.

Together with the Turing machine and counter-machine models, the RA-machine and RASP-machine models are used for computational complexity analysis. Van Emde Boas (1990) calls these three together with the pointer machine, "sequential machine" models, to distinguish them from "parallel random-access machine" models.