Microprocessor Report, July 11, 1994


Western Design Center (WDC) and its founder, Bill Mensch, played a critical factor in the development and release of the Apple IIc and IIGS. It was Mensch who spearheaded a re-creation of the 6502 processor into the lower-powered 65c02, which included the additional assembly language opcodes that made it possible to “crunch” the ROM code in the Apple IIc small enough to fit in its additional built-in functions without excessively complicating the design or coding. His further development of the 16-bit 65816 made it possible to create the more powerful Apple IIGS that still had nearly perfect backward compatibility with older Apple II software.

The 6502 was originally designed by MOS Technology, a company started by some ex-Motorola engineers who wanted to build a “better 6800”. Their first chip, the 6501, was too similar to the Motorola 6800 (although it required some additional support chips to work the same as the 6800, it was pin-compatible, used a few similar op-codes, etc), and Motorola sued to prevent them from selling it. After they made some changes in the design, the 6502 was born. Two other companies who later built and supplied 6502 processors were Synertec (the SY6502) and Rockwell International (R6502).

Some early computers that used the 6502 included:

  • KIM-1 – A single-board computer sold by MOS Technology. It had a hexadecimal display and a hex keypad for data entry. It was intended as a chip evaluation board for hardware developers that were interested in using the 6502. It sold for only $275, when the Altair cost much more, and was a hit with the hobbyist crowd.
  • SYM-1 – built by Synertec; the same type of hex display and keypad design as the KIM-1.
  • AIM-65 – built by Rockwell; it was a little more complete, with an alpha keyboard, a 40-character alpha LED display, and 40-character thermal printer.

(Click this link for more information, historical and otherwise, about the KIM-1 and SYM-1).

All three used a common cassette storage format and had similar I/O ports.

The Article

When I attended KansasFest (an annual Apple II developers and enthusiast’s gathering) in 1995, Bill Mensch was one of the featured speakers. He passed out a reprint of an article published in Microprocessor Report in its July 11, 1994 issue, that discussed the various products made by WDC, both old and new, as well as a historical background of the 6502. I was able to obtain permission from Micro Design Resources, the company that publishes this newsletter, to reproduce here the sidebar part of the article that dealt specifically with that history.

The following introduction to the article is also pertinent and is quoted here:

The [6502] that was at the heart of the Apple II and Commodore 64, among others, was passed from MOS Technology to Synertek to Rockwell, and it might have faded entirely from the microprocessor scene if it were not for one determined individual — William D. Mensch, Jr. — and his tiny company, The Western Design Center (WDC). In 1983, WDC extended the architecture of the 65C02 to 16 bits with the 65C816, which was used in the Apple IIGS. A 32-bit extension, the 65C832, has been discussed for years but remains on the drawing board. Most recently, WDC has crafted a single-chip microcomputer, the 65C265, around the 65C816 core. The 65C816 is at the heart of the Super Nintendo, and Franklin Electronic Publishing uses the 65C02 in its Language Translator and the 65C816 in its Digital Book System.

The 6502’s Long Path to The Western Design Center
July 11, 1994
Michael Slater
(Copyright 1994 In-Stat/MDR, a Cahners Group)

The 6502 has followed a long and twisted path to its current home at WDC. It was created by a group of engineers who had worked at Motorola on the 6800 and its support chips but became disenchanted with their management, which wouldn’t support the products the group wanted to pursue. In August 1974, a group of eight engineers and marketers, including Bill Mensch and Chuck Peddle, left Motorola to work for MOS Technology. MOS Technology was, at the time, the world’s largest manufacturer of calculator chips, and it decided to move up to microprocessors.

Just over a year after the ex-Motorola team joined the company, MOS Technology premiered its first microprocessors — the 6501 and 6502 — at Wescon in the fall of 1975. The architecture bore a striking resemblance to the 6800, with several enhancements, but it was not binary-compatible. The 6501 was pin-compatible with the 6800, whereas the 6502 added the innovation of an on-chip clock generator, which eliminated the need for the two-phase clock input but gave the chip a different pinout. Motorola promptly sued MOS Technology, and an out-of-court settlement was reached in which MOS Technology agreed to take the 6501 off the market but was free to sell the 6502.

The 6502 found its greatest ally in Steve Wozniak, who was just designing the Apple-1. It was selected over its major competitors, the 6800 and 8080, not for any technical reason but simply on price: At a time when Motorola and Intel were charging $300 each for samples, MOS Technology chose to enable widespread experimentation by offering small quantities for the high-volume price of $25. For a couple of kids who were selling off possessions to raise money to build a computer, this was an overwhelming advantage.

Despite its success with the 6502, MOS Technology ran into financial trouble. One of its biggest customers was Jack Tramiel’s Commodore, a leading calculator maker and creator of the Commodore Pet and Commodore 64, two successful 6502-based computers; Commodore acquired MOS Technology in 1976. Commodore later decided to get out of the semiconductor business, bringing to an end the first supplier of 6502 chips.

In the meantime, the 6502 had been licensed to Synertek and Rockwell. Synertek ultimately went out of business, leaving Rockwell as the only supplier of the original NMOS 6502. Rockwell used a derivative of the 6502 as a DSP in its 2400-baud modem chip set.

In 1977, Bill Mensch left MOS Technology (then a division of Commodore). After a stint at the consulting firm Integrated Circuit Engineering, reverse-engineering commercial microprocessors, he decided to start his own design company–at first, with an exclusive agreement to create chip designs for MOS Technology.

After designing a low-power calculator chip for MOS Technology, Mensch decided that he wanted to create a low-power, CMOS 6502 as his next project, but MOS Technology wasn’t interested. He approached Rockwell, GTE, Synertek, and Mitel as well, but they all turned him down. In early 1981, he decided to design it on his own.

Working alone and without outside funding, Mensch designed the 65C02, adding a few new instructions while he created the low-power CMOS design. The chip has a different microarchitecture than the original 6502, using PLA-based microcode for instruction sequencing.

After completing the design, he went back to the same companies, offering to license it to them. This time, both GTE and Rockwell accepted, and Commodore promptly sued for theft of trade secrets. In another out-of-court settlement, Commodore settled in return for being granted the rights to the 65C02 for internal use at half the standard license fee. Synertek also licensed WDC’s 65C02.

(Reprinted with permission of Micro Design Resources)

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