Editing Register machine (section)

==Overview==
The register machine gets its name from its use of one or more "[[Processor register|register]]s". In contrast to the tape and head used by a [[Turing machine]], the [[model]] uses '''multiple uniquely addressed registers''', each of which holds a single non-negative [[integer]].

There are at least four sub-classes found in the [[literature]]. In ascending order of complexity:
* [[Counter machine]] – the most primitive and reduced [[theoretical]] model of computer hardware. This machine lacks indirect addressing, and instructions are in the [[Finite-state machine|finite state machine]] in the manner of the [[Harvard architecture]].
* [[Pointer machine]] – a blend of the counter machine and RAM models with less common and more abstract than either model. Instructions are in the finite state machine in the manner of Harvard architecture.
* [[Random-access machine]] (RAM) – a counter machine with indirect addressing and, usually, an augmented instruction set. Instructions are in the finite state machine in the manner of the Harvard architecture.
* [[Random-access stored-program machine]] model (RASP) – a RAM with instructions in its registers analogous to the [[Universal Turing machine]], making it an example of the [[von Neumann architecture]]. But unlike a computer, the model is ''idealized'' with effectively infinite registers (and if used, effectively infinite special registers such as [[Accumulator (computing)|accumulators]]). As compared to a modern computer, however, the instruction set is still reduced in number and complexity.

Any properly defined register machine model is [[Turing completeness|Turing complete]]. Computational speed is very dependent on the model specifics.

In practical computer science, a related concept known as a [[virtual machine]] is occasionally employed to reduce reliance on underlying machine architectures. These [[Virtual machine|virtual machines]] are also utilized in educational settings. In textbooks, the term "register machine" is sometimes used interchangeably to describe a virtual machine.<ref name="Abelson-Sussman_1996"/>