6501

The 6501 is an 8-bit microprocessor introduced by MOS Technology in 1975. It is a Motorola 6800-compatible CPU, designed to use the same instruction set, registers, and bus interface as the 6800, and to fit in a similar 40-pin DIP package. This compatibility allowed some 6800-based systems to be easily adapted to the 6501 without extensive redesign, effectively enabling drop-in replacement in boards that used the 6800 bus.

Technical overview: The 6501 executes the 6800 instruction set and architecture, including the general-purpose registers and addressing modes used by the 6800. It is intended to operate on the same two-phase clocking scheme as the 6800, and it employs an 8-bit data path with a 16-bit address space. Because of the bus compatibility, designers could reuse existing software and peripheral architectures developed for the 6800.

Development and licensing: The 6501 was developed in the mid-1970s as part of MOS Technology’s early push into microprocessors. Its pin-for-pin compatibility with the 6800 led to legal friction with Motorola, which held patents on the 6800 architecture and bus. The dispute contributed to MOS pivoting away from 6800-compatible designs in favor of the 6502, a cheaper and more flexible successor with a different pinout to avoid patent issues.

Legacy and impact: The 6501 itself saw limited adoption and was soon eclipsed by the 6502, which was introduced around the same period but with redesigned pinout and improved cost structure. The 6501 is often noted for its role in the transitional period of 8-bit microprocessor development and for illustrating the competitive dynamics that helped shape the 6502’s eventual dominance in early home computers and game consoles.