MC6800

The MC6800 is an 8-bit microprocessor introduced by Motorola in 1974. It was one of the first widely available 8-bit CPUs and played a significant role in early embedded systems and some microcomputer designs. The device uses an 8-bit data bus and an 16-bit address space, allowing access to up to 64 kilobytes of memory. It operates with a two-phase, non-overlapping clock and requires external clock generation circuitry. The original parts were built with NMOS technology and could run at relatively modest clock speeds of around 1 MHz, with later variants improving performance.

Architecturally, the MC6800 provides two 8-bit accumulators, A and B, together forming a 16-bit D register. It also has a 16-bit program counter and a 16-bit stack pointer, along with a status register that carries condition flags. The instruction set supports a range of arithmetic, logic, data movement, and control-flow operations. A variety of addressing modes—direct, indirect, indexed, and relative—enable flexible memory access.

The MC6800 ecosystem included several companion devices that extended its capabilities. The 6820 Peripheral Interface Adapter (PIA) was a common I/O companion, enabling straightforward interfacing to keyboards, displays, and other peripherals. The 6850 or related interface chips provided serial communications. Motorola later expanded the lineup with microcontroller derivatives such as the 6801 and 6803, which integrated RAM, ROM, and I/O on a single chip, enabling single-chip microcontroller designs.

Impact and legacy of the MC6800 include its influence on subsequent Motorola CPUs and its role as a competitor to other early microprocessors such as the Intel 8080 and Zilog Z80. Although gradually superseded by more integrated and higher-performance families, the MC6800 left a lasting imprint on the development of embedded microprocessors and microcontrollers.