Motorola 6800

Motorola 6800. The Motorola 6800 is an 8-bit microprocessor introduced by Motorola in 1974. It was a major competitor to early chips like the Intel 8080 and played a foundational role in the development of personal computers and embedded systems. Its architecture influenced numerous subsequent processors and established Motorola as a key player in the semiconductor industry.

History and development

The 6800 was developed by a team led by Tom Bennett at Motorola's Austin, Texas facility, with design work beginning in 1971. The project aimed to create a general-purpose CPU that could be used with a minimal number of support chips, contrasting with more complex contemporaries. A key innovation was the use of a single +5-volt power supply, which simplified system design compared to rivals requiring multiple voltages. The chip was formally announced in March 1974, with Motorola Semiconductor Products Inc. handling its marketing and production. Early engineering samples were provided to companies like Tektronix and NCR, and full production commenced later that year. The development team included several engineers who would later leave to found MOS Technology, creator of the rival 6502.

Architecture and features

The 6800 architecture features a 16-bit address bus capable of directly accessing 64 KB of memory and an 8-bit data bus. It has a relatively orthogonal instruction set with 72 instructions and seven addressing modes, including indexed addressing which proved highly useful. The processor contains two 8-bit accumulators and a 16-bit index register, along with a 16-bit stack pointer and program counter. A significant feature was the inclusion of a memory-mapped I/O model, eliminating the need for separate I/O instructions. The CPU also featured a non-maskable interrupt (NMI) and a vectored interrupt system, enhancing its real-time control capabilities. Its microcode was stored in a ROM internal to the chip, controlling the execution of instructions.

Variants and derivatives

Motorola produced several direct variants, including the 6802, which integrated a 128-byte RAM and an internal clock oscillator, and the 6808, a cost-reduced version. The 6801 and 6803 were enhanced microcontrollers that merged the CPU with RAM, ROM, and a serial communication interface on a single chip. The most significant architectural derivative was the improved Motorola 6809, which offered enhanced performance while maintaining software compatibility. Outside Motorola, the architecture was second-sourced by companies like AMI and Hitachi. Furthermore, the fundamental design inspired the creation of the Motorola 68000 family of 16/32-bit processors, which would become hugely successful in systems like the Apple Macintosh and Commodore Amiga.

Applications and impact

The 6800 found widespread use in early personal computers, such as the SWTPC 6800 and the MITS Altair 680b. It was also employed in arcade games, including early titles from Atari, Inc., and became a staple in industrial control systems and automotive electronics. The processor was central to many development systems and single-board computers used in engineering education. Its design philosophy influenced a generation of engineers and cemented the importance of a simple, unified voltage supply in microprocessor design. The 6800's architecture served as a direct precursor to the immensely popular Motorola 68000 series, which dominated certain computing sectors throughout the 1980s.

Technical specifications

The original 6800 was manufactured using an NMOS process and initially contained approximately 4,100 transistors. It was offered in speed grades from 1 MHz to 2 MHz, with a typical instruction execution time of 2 microseconds. The chip was packaged in a 40-pin dual in-line package (DIP). It required a single +5V power supply and consumed roughly 1.5 watts. The instruction set supported binary arithmetic in both two's complement and unsigned formats, as well as binary-coded decimal (BCD) arithmetic. Its interrupt response time was a critical parameter for control applications, with the processor requiring a defined number of clock cycles to service an interrupt request. Category:Microprocessors Category:Motorola microprocessors Category:1974 introductions