Stored program paradigm

One of the most important breakthroughs in computing was the stored-program concept, proposed by John von Neumann and colleagues in 1945 in the design of the EDVAC computer. The idea was simple but transformative: both instructions and data should be stored together in the same memory. This allowed the CPU to fetch instructions sequentially and made programs flexible; programs could be modified, moved, or even generated by other programs, since they were simply data in memory.

The Von Neumann architecture built on this concept. It describes a computer model in which a single memory holds both instructions and data, and the CPU executes instructions sequentially from that memory. This model underlies virtually all modern general-purpose computers.

However, the Von Neumann model introduces a limitation known as the Von Neumann bottleneck. Because both instructions and data share the same memory and bus system, the CPU cannot fetch an instruction and read or write data at the same time. As CPUs became faster, memory access became the limiting factor in overall performance.

Some systems instead use the Harvard architecture, where instructions and data are stored in separate memories with separate access paths. This eliminates the bottleneck but increases hardware complexity—because control systems must coordinate access across two memory systems rather than a single one. Harvard architectures are common in embedded systems such as microcontrollers, where efficiency and predictability are more important than flexibility. The reduced scope and function of these systems—usually for very particular purposes—allows designers and engineers to pare things down in ways we cannot for general-purpose computers. This simplicity of design often offsets some of the additional complexity of having to manage two separate memory stores.

Adapted from "Patterson and Hennessy, Computer Organization, ARM edition" by Surya Malik and Clayton Cafiero.

No generative AI was used in writing this material. This was written the old-fashioned way.