A Request for Information about the 6502
July 29, 2019
Dear Mr. MENSCH,
I am a 62 year old IBM/COBOL programmer, writing from New Delhi, India.
I am huge fan of computing history and specially the 6502. I believe it was the gunpowder that set off the PC revolution.
However, there is one HISTORICAL question whose answer I cannot find anywhere. Please illuminate it.
What did you/Mr. Peddle in MOS do that improved the yield of the 6502 so dramatically, that you could sell the 6502 for $25 while Motorola/Intel were selling for $300?
I know that you changed the packaging from ceramic to plastic. But is that the whole story? I believe there is more to it. For the sake of historical completeness and engineering insight, can you please enlighten me?
Best Regards,
Ashvini
New Delhi, India
Hi Ashvini,
Thanks for interest in the 6502 technology and thanks for your question.
The better/higher yield were due to the following main factors:
1) The NMOS depletion mode load transistor process while the Intel and Motorola processes used bigger and more enhancement mode load devices. The result was smaller and less transistors on the 6502 as compared to the Intel 8080 and Motorola 6800.
2) MOS Technology had their own mask shop much like Motorola; that said, the mask shop at MOS had a proprietary process of “spot knocking” or manually patching up the holes in the reticles and projection masks so there were no holes. This created perfect masks that could be used over and over again as long as the mask was not damaged. Motorola and Intel used contact masks which had defects especially after one use.
3) The logic of the 6502 instruction set was simpler and used less cycles and less registers; therefore a much smaller die, 2-4 times smaller die.
4) The ratio between the switch and load transistors was more forgiving so much so that the loads could have been double implanted with the depletion dose and still fully operate. It was accidently double implanted and those chips ran twice the speed, 10MHz when the standard process ran typically at 4-5MHz. The result of having a more forgiving design was that process variations didn’t affect the yields as much as other designs. I chose the process and the circuit designs which for the 6502 resulted in my designing the non-overlapping two phase clock generator that used either a crystal (accurate) or RC (less accurate but lower cost) source. The 6501 used an external more expensive two phase non-overlapping clock generator like the Intel 8080 and Motorola 6800. Intel also used three power supplies (5 and +12 and -12 volt supplies) which made the Intel processor even more expensive to design with. The 6501/6502 used one 5 volt supply as did the 6800.
5) Another big reason the 6502 won the early PC battles was because of its pipelined and addressable register architecture. The addressable register architecture, in “zero page”, was quite useful in creating, in memory, registers for efficient and useful code for, as an example, double precision floating point libraries that was used on the Apple II.
I hope this helps you understand why the 6502 sold for $25 while the Intel 8080 and Motorola 6800 sold for approximately 10 times more.
Best,
-Bill
Respected Sir,
Thank you so much for your detailed answer, I loved it.
Have a great day.
Ashvini
New Delhi, India.
Ashvini,
You are very welcome.
Best,
-Bill