Tuesday, May 5, 2009

MOS Technology

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Not to be confused with Mostek.
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.
1 History
1.1 Mask fixing
1.2 6502 family
1.3 Commodore Semiconductor Group
1.4 GMT Microelectronics
2 Products
3 Notes
4 External links
MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor) was 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 Business Machines, who bought practically all of their supply for their line of calculators.[1]
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.
Mask fixing
In addition, MOS had a secret weapon: the ability to "fix" its masks.[2] 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.
If a chip design with five design flaws results in a mask with ten flaws in total, there is no point in making another mask 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. In the late 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 hat 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.
6502 family
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 1 MHz in September 1975 for a mere $25. It was essentially identical to the 6501, differing only in pin layout. 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.

Image of the circuit board of a Commodore 64 showing some important MOS Technology circuits: the 6510 CPU (long chip, lower left) and the 6581 SID (right). The production week/year (WWYY) of each chip is given below its name.
The 6502 was so cheap, that many people believed it was a scam when MOS first showed it 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...(and so on)

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