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Image of the innards of a Commodore 64 showing some important MOS Technology circuits: the 6510 CPU and the 6581 (SID).
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.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 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 of the USA) 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.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 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 1970s, 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 motherboardA motherboard also known as mainboard logic board or systemboard is the central or primary circuit board making up a complex electronic system, such as a computer. A typical computer is built with the microprocessor, main memory, and other basic component 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 8080The Intel 8080 was an early microprocessor designed and manufactured by Intel. The 8-bit CPU was released in April 1974 running at 2 MHz, and is generally considered to be the first truly usable microprocessor CPU design. Description The Intel 8080 was th and Zilog Z80The Zilog Z80 is an 8-bit microprocessor designed and manufactured by Zilog from 1976 onwards. It was widely used both in desktop and embedded computer designs, and is one of the most popular CPUs of all time. Although Zilog made several attempts to move 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 InternationalRockwell International was the ultimate incarnation of a series of companies under the sphere of influence of Willard Rockwell, who had made his fortune after the invention and successful launch of a new bearing system for truck axles in 1919. Primary amo, GTE, Synertek and Western Design Center (WDC)The Western Design Center (WDC maker of microprocessors, microcontrollers, and related support chips, was founded in 1978 by co-holder of the MOS Technology 6502 patent, Bill Mensch, himself a former MOS Technology employee. WDC is located in Mesa, Arizon.
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 6507The 6507 is an 8-bit microprocessor from MOS Technology, Inc. It is a "cut down" version of their popular 6502. To reduce costs the 6507 included only thirteen address pins instead of the 6502's sixteen. This allowed the 6507 to address 8 KB of memory, at, 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.
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