CPUs: Motorola 6800 and 6809

The personal computing revolution started in 1974 with the 2 MHz Intel 8080, Intel’s first 8-bit CPU capable of addressing 64 KB of memory. (The earlier 8008 from 1972 could only address 16 KB.) But there was a parallel path, a new CPU family that Motorola launched in 1974.

Motorola 6800 CPUThe 1 MHz Motorola 6800 was also an 8-bit CPU capable of using 64 KB of system memory. Although the 6800 and 8080 were designed more or less simultaneously, they had some different goals. The 8080 had more registers (which were 16-bit but each could be used as two 8-bit registers), while the 6800 had fast access to the first 256 bytes of RAM and supported direct memory access, which allowed another device (such as a disk controller) to write data directly to system memory.

Despite the difference in clock speed, the 8080 and 6800 performed comparably because the 8080 instructions took more cycles to process. Motorola later produced a 2 MHz version of the 6800.

Many members of the Motorola 6800 design team left for MOS Technology, where they designed the 6501 and 6502 CPUs. The 6501 was short-lived, because Motorola sued over it being pin-compatible with its 6800 CPU. The 6502 went on to power many different home computers.

Early 6800-based Computers

The first 6800-based computer kits arrived in late 1975, starting with the Sphere at $650 with 4 KB of memory and the ability to display 16 lines of 32 characters on a TV screen or CRT monitor. Southwest Technical Products Corporation announced its SWTPC 6800 Computer System in November 1975, and it would go on to be the most successful 6800-based computer. MITS, famous for its Altair 8800 (based on the Intel 8080), also made the MITS Altair 680, which took a back seat to the 8800.

Motorola 6809: More Power

Motorola 6809 CPUWith the 1978 introduction of the 6809, Motorola streamlined the 6800’s instruction set and added some 16-bit features, making it a big step forward from the 6800. Another benefit is that the 6809 was easier to program than the 6800 or 6502. The 6809 was initially available in 1 MHz, 1.5 MHz, and 2 MHz versions, and Hitachi would later produce even faster versions.

Motorola engineers considered the 6809 so well designed that there was no further room for improvement.

Tandy Color Computer 3The 6809 is best known for its use in the TRS-80 Color Computer, but it was also used by Acorn, Fujitsu, Canon, and Dragon, among others. Apple originally planned to build the Macintosh around the 6809 – until the design team determined that what they wanted to do could not be accomplished within 64 KB of memory. Then they moved to the Motorola 68000 CPU.

Commodore used the 6809 alongside the 6502 in its dual-CPU SuperPET, and the 6809 also made its way into some arcade games.

Hitachi Pushes the Envelope

Hitachi was a licensee of the 6809, but its CMOS 6309 was more than a clone of the 6809. The 6309 was available at higher clock speeds (3.5 MHz), had more instructions and registers, and generally ran 30% faster than the 6809 when used in native 6309 mode. It was very popular as an upgrade for the TRS-80 Color Computer.

Further Reading

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