The view from above
Mensch, a senior life member of the IEEE, divides his time between Arizona and Colorado, but residents of the Northeastern United States will have the opportunity to see him as a keynote speaker at the Vintage computer festival in Wall, NJ, the weekend of October 8. Before Mensch appeared, The institute caught up with him via Zoom to talk about his career.
This interview has been condensed and edited for clarity.
The institute: What attracted you to engineering?
Bill Mensch: I went to Temple University [in Philadelphia] on the recommendation of a guidance counselor. When I got there I found out that they only had an associate’s degree in engineering technology. But I didn’t know what I was doing, so I thought, let’s finish that associate degree. Then I got a job [in 1967] as a technician at [Pennsylvania TV maker] Philco-Ford and noticed that engineers made about twice as much money. I have also noticed that I help engineers understand what Motorola was doing in high voltage circuits, which meant Motorola was the leader and Philco was the follower. So I went to the University of Arizona, near where Motorola was, got my engineering degree [in 1971] and went to work for Motorola.
IT: How did you develop the 6502?
BM: Chuck peddle approached me. He arrived at Motorola two years after I started. Now this has not been written anywhere to my knowledge, but I think its intention was to loot Motorola for the engineers. He worked with me on the Peripheral Interface Chip (PIA) and got to see me in action. He decided that I was a selfish young engineer who was just the right kind to go with his ego. So Chuck and I formed a kind of partnership. He was the systems engineer and I was the semiconductor engineer. We tried to start our own business [with some other Motorola engineers] and when that didn’t happen we joined a [semiconductor design] company, called MOS Technology, in Pennsylvania in 1974. This is where we created the 6501 and 6502 [in 1975], and I designed the I / O chips that came with it. The intention was to [develop a US $20 microprocessor to] compete with the Intel 4040 microcontroller chipset, which was selling for around $ 29 at the time. We weren’t trying to compete with the 6800 or the 8080 [chips designed for more complex microcomputer systems].
IT: The 6502 has become the basis of many microcomputer systems, and if you look contemporary programmer books, they often talk about the quirks of the 6502’s architecture and instruction set compared to other processors. What motivated these design decisions?
BM: Orgill Stem and I had completed designs for a few microprocessors prior to the 6501/6502. In other words, Rod and I already knew what worked in an instruction set. And lower cost was the key. So we looked at the instructions we really needed. And we figured out how to have addressable registers using zero pages [the first 256 bytes in RAM]. You can therefore have one byte for the operation code and one byte for the address, and [the code is compact and fast]. There are limitations, but compared to other processors zero pages was a big deal.
There is a love for this little processor that is undeniable.
IT: Many pages of these programming books are devoted to explaining how to use the General Purpose Interface Adapter (VIA) chip and its two I / O ports, built-in timers, a serial shift register, etc. Why so many features?
BM: I had worked on the previous PIA chip at Motorola. This meant that I understood the needs of real systems in real world implementations. [While working at MOS] Chuck, Wil Mathis, our applications manager, and I were eating at Arby’s one day, and we talked about doing something beyond PIA. And they’d say, “We’d like to put some timers in there. We would like a serial port ”, and I said,“ Okay, we’re going to need more register select lines. And our notes are on an Arby napkin. And I went and designed it. Then I had to redesign it to make it more compatible with the PIA. I also made some changes to Appledemand. What’s interesting about the VIA is that it’s the most popular chip we sell today. I am discovering more and more how it has been used in different applications.
IT: After MOS Technology, in 1978 you founded The Western Design Center, where you created the 65C816 processor. The creators of the ARM processor credit a visit to WDC as giving them the confidence to design their own chip. Do you remember that visit?
BM: Strongly! Sophie Wilson and Steve Furber visited me and told me about the development of a 32-bit chip. They wanted to go beyond what Apple was supposed to be doing. But I was just finishing the ‘816, and I didn’t want to change horses. So when they [had success with the ARM] I encouraged them because it was not something I wanted to do. But I left them with the idea of, “Look, if I can do it here… there are two of you; there is one among me ”.
BM: I’m excited about what’s going on right now. It’s more exciting than ever. they just gave them to me 6502 flexible printed with thin films by Pragmatic! Our chips are in IoT devices, and we have new educational boards coming out.
IT: Why do you think the original 65x series is still popular, especially among the people who are building their own personal computers?
BM: There is a love for this little processor that is undeniable. And the reason is, we packaged it with love while we were designing it. We knew what we were doing. Rod and I knew this from our previous experience with the Olivetti processor and other chips. And from my work with the I / O chips, I knew [how computers were used] in the real world. People want to work with 65x chips because they are accessible. You can trust the technology.
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