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MOS Technologies

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

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Company History

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 CBM), 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 an IP firm 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) so that was that.

The team of four design engineers was headed by Chuck Peddle and included other designers such as 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 much faster, up to four times.

In addition, MOS had a secret weapon, the ability to "fix" their 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 particular mask ends up with 10 flaws, there's no point in making another because it will have the same five design flaws, and some other set of five copying flaws. So you simply build with what you have, and throw away broken chips. At the time in the 1970's, this mean throwing away 70% or more of the completed chips. The price of a chip is largely defined by how many work, the yield, so improving this number can lower the price dramatically.

MOS 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. Their production lines typically reversed the numbers others were achieving, even the early runs of the new CPU design were achieving a success rate of 70% or better. This meant that not only were their designs faster, they cost much less as well.

When the 6501 was announced, Motorola launched a lawsuit almost instantly. 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, Intel 8080 and Zilog Z80 series, but cost much less and was easier to work with. Although it didn't have the advantage of being able to be used in existing Morotola 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 considered it to be some sort of scam when it was first shown at a trade show in 1975. They weren't aware of MOS's masking techniques and when they calculated the price per chip at normal yield rates it didn't 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. In fact this simply 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 and Synertek.

A number of different versions of the basic CPU, known as the 6503 through 6507, were offered in 28-pin packages for lower cost. The various models removed signal or address pins. Far and away the most popular of these was the 6507, which was used in the Atari 2600 and in Atari disk drives. The 6504 was sometimes used in printers. MOS also released a series of similar CPUs using external clocks, which added a "1" to the name in the 3rd digit, as the 6512 through 6515. These were useful in systems where the clock support was already being provided on the motherboard by some other chip. The final addition was the "crossover" 6510, used in the Commodore 64, with additional I/O ports.

Commodore Semiconductor Group

However successful the 6502 was, the company itself was having problems. At about the time the CPU was released the entire calculator market collapsed, and MOS's only existing products stopped shipping. Soon they were in serious financial trouble. Rescue came in the form of Commodore, who bought the entire company in a stock trade, on the condition that Chuck Peddle would join Commodore as chief engineer. The deal went through, and while the firm basically became Commodore's production arm, they continued using the name MOS for some time so that manuals wouldn't have to be re-printed. After a while MOS became Commodore Semiconductor Group, CSG.

MOS had also designed a simple computer kit called the KIM-1, primarily to "show off" the 6502 chip. At Commodore Peddle convinced the owner, Jack Tramiel, that calculators were a dead-end, and that home computers would soon be huge. A re-packaged KIM with a new display driver and keyboard became the Commodore PET computer.

However the original group appeared to be even less interested in working for Jack Tramiel than they had for Motorola, and the team quickly started breaking up. One result was that the newly-completed 6522 (VIA) chip was left undocumented for years.

Bill Mensch left MOS even before the Commodore takeover, and moved home to Mesa, AZ from MOS's Norristown, PA. After a short stint consulting for a local company called ICE, he set up the Western Design Center (or WDC) in 1978. As a licensee of the 6502 line, their first products were CMOS versions of the 6502 (embedded inside a microcontroller called the 65C150) which required less power. But then he expanded the line greatly with the introduction of the 65816, a fairly straightforward 16-bit upgrade of the original 6502 that could also run in a 8-bit mode for compatibility. The design of the similar-in-concept 32-bit 65832 CPU was completed, but not put into production (to date). Since then they have moved much of the original MOS catalog to CMOS, and the 6502 continues to be a popular CPU for medical equipment and card dashboard controllers.

After Commodore's bankruptcy in 1994, the Commodore Semiconductor Group (the former MOS Technology) was bought by its former management for about $4.3 million, plus an additional $1 million to cover miscellaneous expenses including EPA liens. Dennis Peasenell became CEO. In 1995, the company, operating under the name GMT Microelectronics, reopened a troubled facility in Norristown, Pennsylvania that Commodore had closed in 1992. By 1999 it had $21 million in revenues and 183 employees. However, in 2001 the EPA shut the plant down. GMT ceased operations and was liquidated.


Information on MOS's "secret" are from a phone interview with Bill Mensch in 2002. Mask-fixing is now widespread.

Another theory on the calculator line drying up is somewhat more conspiratorial. It states that Commodore deliberately overbought MOS's chip line to monopolize it, and warehoused the extras. Then, with several months worth stored, they stopped buying anything and MOS's sales died. This forced MOS to sell to Commodore.


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This article (or an earlier version of it) contains material from FOLDOC, used with permission.

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