Saturday, May 19, 2018

The MOS 6502 CPU microprocessor

In the early '70s, a group of engineers from Motorola was convinced that it was possible to obtain a cheap microprocessor if a simple design and an improved manufacturing process were raised with respect to the standards of the time. Without the support of the company, this group would create one of the families of chips that would be the catalyst for the revolution of home computing, it is the 6502 and its successors, the main engine of the first Atari and Apple machines, between others.

Chuck Peddle was one of the leading engineers in the development of the Motorola 6800 processor, in his meetings with important industrial clients, he realized that the technology was appreciated, but too expensive to be used in a massive way. In conversations with his clients, he concluded that the cost of the processor had to go down from USD $ 300, which cost 6800 to only USD $ 25 at the time, a conclusion that did not make Motorola managers who had no motivation at all happy. to look to reduce the price of a technology that already had good sales.

Chuck Peddle and his work

For Chuck and other engineers it was an interesting technical challenge and without the support of Motorola they began to work on the necessary changes to improve their processor. On the one hand, the team was unhappy with Motorola's lack of support, and on the other hand the company was annoyed by Chuck's and his team's stubbornness, and finally the straw that broke the camel's back was Motorola's formal request to halt the project. that made Chuck and his team decide to leave the company, to continue working on their microprocessor on their own.

6502 CPU chip (C)


The group joined a low profile company called MOS, where they reached an agreement to participate in the profits that could be obtained with the new microprocessor, an agreement that from the MOS point of view was convenient due to the low profitability that meant in that time the production of these chips. This agreement, together with the mass resignation of Motorola was a demonstration of the confidence that the team of engineers had in the product they were developing.

The first processor of the 6500 series was 6501. An unprecedented fact that surrounded the development of this chip was that Chuck designed it completely by hand and it worked on the first attempt, when the normal thing in those years was to go through a process of over 10 failed attempts This processor was quite scarce since it was not made in order to be sold, but as a way to show Motorola how wrong they were, and that a useful and inexpensive microprocessor could be built.

One of the factors that influenced the reduction of production costs was an innovation of these engineers regarding the manufacturing process. In those times, the failure rate in production was 70%, that is, only 7 of every 10 processors manufactured went straight to the trash. This happened mainly because the process consisted of creating a large mold of the processor and applying reductions until reaching the real size, in these reductions errors always occurred and the engineers in MOS found a way to apply corrections to the mold in each reduction, achieving with this change a success rate of 70%, that is, 7 out of 10 processors were perfect.

The 6501, a Motorola clone?

The 6501 was fully compatible with the Motorola 6800 boards and could be used as a direct replacement at hardware level, which motivated a Motorola claim for alleged patent infringement of its former engineers. Supposed because in fact what was at stake was the intellectual property, since it was applying the knowledge of technologies that in fact were not patented. Even so an agreement was reached with Motorola and a sum of about USD $200,000 was paid to end the lawsuit.

The problem with Motorola caused the creation of the 6502, it was a 6501 but incompatible with the hardware designed for the Motorola 6800. The 6502 debuted at WestCon 1975 at the exact price of USD $ 25 that had been proposed. At first the attendees thought that it was some kind of scam, since it was not conceived to create such a cheap processor, but on the same day Motorola and Intel lowered the price of their 6800 and 8080 processors from USD $ 179 to only USD $ 69, validating the 6502 that was sold by the hundreds.

In the coming years Commodore, who was still in the calculator business, was going through financial difficulties as the business was going down and the competition was getting stronger. As a strategy to change the direction of the business, they bought from MOS, which was also struggling but had the technology they needed.

For Chuck Peddle the microprocessor business was geared to the industrial world, but while visiting different companies to try to sell the 6502 they told him that a couple of guys were trying to use their processor to build a personal computer. The fact seemed nice and agreed to go to help them to his garage, it was nothing more and nothing less than Steve Jobs and Steve Wozniak who worked on his first Apple computer. Chuck thought about introducing the 6502 into the industries and he did not suspect that a big market was in personal computers. Not in a million years! In his own words.

The 6502 is an 8-bit processor, which means that all its internal operations handle numbers between 0 and 255, on the other hand it has a 16-bit address bus that allows it to access up to 64KB of memory. The 6502 normally operated with a clock between 1 and 2 Mhz but given the same clock frequency it was capable of running much faster than the Motorola 6800, thanks to its clever design that reduced the number of clock cycles needed to execute an instruction .

Among the computers that were built around the 6502 are the Commodore PET, VIC-20, Apple I and II, BBC Micro and the entire line of Atari 8 bit computers such as the XL, XE, 400, 800, but also can be found in other types of devices, such as the Commodore 1541 disk drive for Commodore 8 bit computers. In the pioneering Atari 2600 console, an economical version of the 6502 known as 6507 was used, a version that had fewer pins and could only access 8KB of memory. The Commodore 64 also used a modified 6502, it was the 6510 where it added simple features that were usually implemented with additional circuitry.

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