Editing Von Neumann architecture (section)

==History==
The earliest computing machines had fixed programs.  Some very simple computers still use this design, either for simplicity or training purposes.  For example, a desk [[calculator]] (in principle) is a fixed program computer.  It can do basic [[mathematics]], but it cannot run a [[word processor]] or games.  Changing the program of a fixed-program machine requires rewiring, restructuring, or redesigning the machine.  The earliest computers were not so much "programmed" as  "designed" for a particular task.  "Reprogramming"—when possible at all—was a laborious process that started with [[flowchart]]s and paper notes, followed by detailed engineering designs, and then the often-arduous process of physically rewiring and rebuilding the machine. It could take three weeks to set up and debug a program on [[ENIAC]].<ref>{{Harvnb|Copeland|2006|p=104}}.</ref>

With the proposal of the stored-program computer, this changed. A stored-program computer includes, by design, an [[instruction set]], and can store in memory a set of instructions (a [[computer program|program]]) that details the [[computation]].

A stored-program design also allows for [[self-modifying code]]. One early motivation for such a facility was the need for a program to increment or otherwise modify the address portion of instructions, which operators had to do manually in early designs. This became less important when [[index register]]s and [[Addressing mode|indirect addressing]] became usual features of machine architecture. Another use was to embed frequently used data in the instruction stream using [[Addressing mode|immediate addressing]].

When von Neumann described the automatic computing systems using different terminology than is typically described with the model. In the ''[[First Draft of a Report on the EDVAC]]'',<ref name ="FirstDraftReport" /> the architecture was composed of "a high-speed memory M, a central arithmetic unit CA, an outside recording medium R, an input organ I, an output organ O, and a central control CC" <ref>{{cite book |last1=von Neumann |first1=Jon |title=Theory of Self-reproducing Automata |date=1966 |publisher=University of Illinois Press |isbn=978-0252727337 |page=10}}</ref>