How does a Von Neumann architecture differ from a Harvard architecture?

Von Neumann architecture uses a single memory and bus for both data and instructions, while Harvard architecture separates them.

Von Neumann architecture, named after the mathematician and physicist John Von Neumann, is a computer architecture based on the 1945 description by him and his colleagues. This architecture is designed with a single, shared memory for instructions (program) and data. This means that the instructions and data use the same memory channels or buses for communication, leading to a bottleneck effect known as the Von Neumann bottleneck. This bottleneck is the limited throughput between the central processing unit (CPU) and memory compared to the amount of memory.

In contrast, the Harvard architecture has physically separate memories and buses for instructions and data. This architecture, originally used in the Harvard Mark I relay-based computer, allows the data and instructions to be fetched simultaneously, eliminating the bottleneck effect experienced in the Von Neumann architecture. This separation provides increased speed and efficiency, making the Harvard architecture particularly useful in real-time signal processing and applications requiring complex computations.

However, the Von Neumann architecture is simpler and cheaper to implement, as it requires fewer hardware resources. It is also more flexible, as the size of the memory does not need to be determined at the time of design. On the other hand, the Harvard architecture, while faster and more efficient, is more complex and expensive to implement. It also lacks the flexibility of the Von Neumann architecture, as the size of the memory for instructions and data must be determined at the time of design.

In practice, many modern computers use a modified Harvard architecture, where the CPU has separate instruction and data caches (small, fast memories) at the first level, providing the speed benefits of the Harvard architecture, but share main memory, providing the cost and flexibility benefits of the Von Neumann architecture. This is often referred to as the Harvard-Von Neumann architecture.