In August 1974, a group of disgruntled Motorola employees left to found a company where they would be able to create and use the chips they designed. While this company was little known outside of the computer industry, the impact it had is beyond belief. This company, the creator of the famed 6502 processor, was MOS Technologies.

MOS Technologies was founded in Norristown, PA by Chuck Peddle, Bill Mensch, and several other ex-Motorola IC designers. With the company in place, they quickly began work on a processor similar to the 6800 chip of their former employer. The resulting chip, the 6501, was fourfold faster than the 6800, similar in design, and was pin-compatable. This did not make Motorola happy, and the latter sued the company in June 1975. (The result of the lawsuit was $20,000USD, even then a weak little sum.)

Meanwhile, in September 1975 MOS released the 6502. At 1MHz, it outran the more complex but slower competition, such as the 6800, Intel 8080, and the Zilog Z80 chips. At $25 per chip, it was the least expensive processor at market, and although it wasn't pin-compatable with the 6800, it was much more popular and all but ran that chip out of town. However, around this time the market collapsed, placing MOS Technologies into financial difficulty.

The next summer, Peddle designed a PC then called the KIM-1, using the 6502 as the heart. This system was licensed by Commodore Business Machines and became the Commodore PET. Meanwhile, Mensch left MOS to be a private consultant. In September of 1976, Commodore announced that they were buying MOS Technologies to keep from being hosed by a betrayal by Texas Instruments. The company, under Commodore, was renamed as Commodore Semiconductor Group (CSG).

Mostly unrelated, in 1977 Western Design Center was founded by Mensch. It licensed the 6502 and has since made thousands of custom 6502 chips for about every product imaginable, from cell phones to kitchen equipment.

With the introduction of the Amiga, MOS/CSG was less important to the Commodore plan. Commodore used Motorola 68000 series processors for all the Amiga computers, but still used their own chips for certain devices.

In 1989, the Environmental Protection Agency listed the Norristown fabrication facility to its hazardous waste site list after finding that toxins were leaking from storage tanks at the facility. This problem would come to haunt them again later. In 1992, the Norristown facility was closed by Commodore, as one of the many cost-saving measures they had been enacting since the mid-eighties. With that, the division declared bankruptcy, but kept the facility neat and tidy in hopes of selling it off to anyone interested. Two years later, it was Commodore which went bankrupt. CSG/MOS was sold to GMT Microelectronics Corporation, which re-opened the facility for production.

From December 1994 until its demise, GMT was in discussions with the EPA about the hazards of Norristown. Finally, in 2001 GMT was shut down by the EPA, and a year later, the EPA began a cleanup of the facility.

A note: The reason for MOS being able to sell chips at about a fifth of market average was due to a technology they used known as mask fixing. Today, this is industry standard practise, but back then it wasn't. And because of that, 70% of fabricated chips were failed and destroyed as unsellable, which naturally raised the price of those that were good. MOS, thanks to fixing its masks (the design that is imprinted on the chip), was able to keep their fail rate much lower, and as a result were able to sell chips at a much lower price than the competition.