MOS Technology

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MOS Technology, Inc., also known as CSG (Commodore Semiconductor Group), was a semiconductor design and fabrication company based in Norristown, Pennsylvania, in the United States. It is most famous for its 6502 microprocessor, and various designs for Commodore International's range of home computers.



MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor) was originally started in 1969 by Allen-Bradley to provide a second source for Texas Instruments (TI) designed electronic calculators and the chips inside them. In the early 1970s TI decided to release their own line of calculators, instead of selling just the chips inside them, and introduced them at a price that was lower than the price of the chipset alone. Many early chip companies were wiped out in the aftermath; those that survived did so by finding other chips to produce. MOS became a supplier to Atari, producing a custom single-chip Pong system.

Things changed dramatically in 1975. Several of the designers of the Motorola 6800 left the company shortly after its release, after management told them to stop working on a low-cost version of the design. At the time there was no such thing as a "design-only" firm (known as a fabless semiconductor company today), so they had to join a chip-building company to produce their new CPU. MOS was a small firm with good credentials in the right area, the east coast of the USA. The team of four design engineers was headed by Chuck Peddle and included Bill Mensch. At MOS they set about building a new CPU that would outperform the 6800 while being similar to it in purpose. The resulting 6501 design was somewhat similar to the 6800, but by using several simplifications in the design, the 6501 would be up to four times faster.

Mask fixing

In addition to a good design, MOS had a secret weapon: the ability to "fix" its masks.[1] Masks are the large drawings of the chip that are photo-reduced to make the pattern from which chips are made – a process similar to photocopying. All masks end up with flaws, both as a result of design problems in the chip itself, as well as side effects from the photo-reduction process. When a chip is made with this mask there is a chance that some of these flaws will end up expressed on the chip. If too many of them are expressed, that particular chip will not work.

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