Von Neumann architecture
The bus of the early single-board devices, such as the Z80 and 6502, was universally a Von Neumann architecture. Program and data memory were accessed via the same shared bus, even though they were stored in fundamentally different types of memory: ROM for programs and RAM for data. This bus architecture was needed to economise the number of pins needed from the limited 40 available for the processor's ubiquitous dual-in-line IC package.