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MOS Technology, Inc., also known as Commodore Semiconductor Group, was a microprocessor and calculator company famous for its 6502 processor.

(Note that, despite the name similarity, MOS Technology is not the same company as Mostek.)

Company history

A better 6800 than the 6800

MOS originally started up to provide a second source for Texas Instruments designed electronic calculators and the chips inside them. They also produced Atari's custom PONG chip for a short time. As the calculator market grew MOS eventually became largely beholden to Commodore International (then Commodore Business Machines), who bought practically all of their supply for their line of calculators.

Things changed dramatically in 1975. Several of the designers of the Motorola 6800 left the company shortly after its release, apparently in disgust. 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 any of their designs. 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.

Lowering the costs by mask fixing

In addition, MOS had a secret weapon, the ability to "fix" its masks. 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 chip will not work.

If a chip design with five flaws results in a mask with ten flaws, there is no point in making another because it will have the same five design flaws plus some other set of five copying flaws. So companies simply built chips with these masks, and threw away broken chips. At the time in the 1970s, this meant throwing away 70% or more of the completed chips. The price of a chip is largely defined by the yield, the measure of how many work, so improving this number can lower the price and raise the gross profit dramatically.

MOS's engineers had learned the trick of fixing their masks after they were made. This allowed them to correct the major flaws in a series of small fixes, eventually producing a mask with a very low flaw rate. The company's production lines typically reversed the numbers others were achieving; even the early runs of a new CPU design--what would become the 6502--were achieving a success rate of 70% or better. This meant that not only were its designs faster, they cost much less as well.

The 6502 family makes its mark

When the 6501 was announced, Motorola launched a lawsuit almost immediately. Although the 6501 was not compatible with the 6800, it could nevertheless be plugged into existing motherboard designs because it used the same arrangement of pins. That was enough, apparently, to allow Motorola to sue. Sales of the 6501 basically stopped, and the lawsuit would drag on for many years before MOS was eventually forced to pay a paltry $200,000 in fines.

In the meantime the 6502 had gone on sale at 1MHz in September 1975 for a mere $25. Due to its speed it outran the more complex and expensive 6800, and Intel 8080, but cost much less and was easier to work with. Although it did not have the advantage of being able to be used in existing Motorola hardware like the 6501, it was so inexpensive that it quickly overran the 6800 in popularity anyway, making that a moot point.

In fact the CPU was so cheap that many people believed it to be scam when the company first showed the 6502 at a 1975 trade show. They were not aware of MOS's masking techniques and when they calculated the price per chip at normal yield rates it did not add up. But any hesitation to buy it evaporated when both Motorola and Intel dropped the prices on their own designs from $179 to $69 at the same show in order to compete. Their moves legitimized the 6502 and by the end of the show all the samples were gone.

The 6502 would quickly go on to be one of the most popular chips of its day. A number of companies licensed the 650x line from MOS, including Rockwell International, GTE, Synertek, and Western Design Center (WDC).

[ Visit the complete Wikipedia entry for MOS Technology ]

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