1.3: Von Neumann and Harvard Architectures

When discussing how memory is accessed at the CPU level, there are two designs to consider. The first is a Von Neumann architecture, and the second is a Harvard architecture. The major difference between the two architectures is that in a Von Neumann architecture all memory is capable of storing all program elements, data and instructions; in a Harvard architecture the memory is divided into two memories, one for data and one for instructions.

For this monograph, the major issue involved in deciding which architecture to use is that some operations have to access memory both to fetch the instruction to execute, and to access data to operate on. Because memory can only be accessed once per clock cycle, in principal a Von Neumann architecture requires at least two clock cycles to execute an instruction, whereas a Harvard architecture can execute instructions in a single cycle.

The ability in a Harvard architecture to execute an instruction in a single instruction leads to a much simpler and cleaner design for a CPU than one implemented using a Von Neumann architecture. For this first monograph a Harvard implementation will be implemented. Later monographs will look at the implementation of the CPU using the Von Neumann architecture.

Figure 1-8: Difference between a Von Neumann and Harvard architecture