The Brains Behind The Brains
An Interview With William Mensch, Microprocessor Pioneer
Selby Bateman, Features Editor
|Implanting a powerful microprocessor in the human body-to assist the blind and deaf and to monitor and stimulate damaged hearts-is just one of chip designer Bill Mensch's current projects. The idea may sound farfetched, but Mensch's credentials are impeccable. He singlehandedly designed the 16-bit 65816 microprocessor that Apple Computer chose as the central processing unit (CPU) for its new Apple IIGS. He's also the father of the Apple IIc's 65C02 processor, and was instrumental in the development of the original 6502 chip. As founder and director of his own company, Western Design Center, the ingenious and independent Mensch is making a name for himself as the wizard of microprocessors. Compute! recently spoke to him about the 65816 chip, the history of the 6502. family, and the future of computers.|
COMPUTE!: How has the design process changed from when you first worked on the 6502 to the development of the 65816-is it a team effort, or is it primarily one or two people?
Well, as it turns out, on the
6502 we had several people involved in the design and the definition
of the product. GTE was involved with it. Apple was not involved with
And who also happen to be brother and sister.
Yeah, brother and sister on top of it. It ends up being a very special thing that we share.
How does the 65816 microprocessor relate to the earlier 6502 chip?
The history of the 6500 series microprocessor starts with the 6502 that was NMOS [one of several versions of MOS] technology. We developed the 65C02 here at Western Design Center, and when we developed the 65C02 we also had the 65816 in mind.
When did you start work on the 65C02?
In 1981. We delivered the first
chips to Apple in the spring of 1982. The 65C02 instruction
enhancement has actually 29 instruction opcodes added to the original
6502 instruction set. [An opcode is a machine language
Which means that the 6502 was quite an expandable chip, relative to the others?
That's right. It was planned originally to have upgrade paths back in 1975 when we developed the product. So, here we are in 1986, and I'm just finishing up a couple more chapters in the original concept, or plan.
And how does that affect the average Apple computer user?
The end user, the guy who buys an Apple II or other products that use the 6502, what they get is the ability to upgrade their system without giving up their software. And when you have a plug-compatible part, it also means that you don't have to give up your hardware.
So, the overall goal of your efforts...?
Our game plan is to upgrade the Apple IIe to a 16-bit capability, and then have the total technology capability to reduce the number of chips in the system or improve the performance of the system, or a combination of both, so that the user ends up with more value for the dollar.
And with no loss of compatibility.
Right-total compatibility. We always have to stay with the compatibility.
If someone wanted to purchase a 65816 directly from you, what would be the cost?
Our price is $95.00 in single quantity. We realized there would be people interested in having single-piece quantities (for the people that have personal computers out there), so we set up a price that is not totally inconsiderate of the user.
COMPUTE!: You've touched on this, but what would you say the chief problem is in developing a compatible chip as you have with the 65C02 and now the 65816?
The chief problem is, if you
start off with the wrong processor, you're always going to be
fighting the system. In other words, if it wasn't for the 6502
original concept, I would have been having a disaster of a time
trying to do it.
How difficult is it to achieve compatibility in a microprocessor?
The chief problem that exists in
maintaining the compatibility going from the 65C02 to the 65816 was
forcing the discipline of total compatibility-meaning the same number
of cycles in the instruction, the same construction of the
instruction, the same architecture of the instruction. That meant
that I had to add some logic in there for emulation.
The clock speed of the 65816-is it about 4 megahertz?
Yes. Our typical product is 4-megahertz. We sell 2-megahertz into the retrofit market, but our typical run-of-the-mill is 4-megahertz. And that's at the 3-micron level. At 2.4 microns, which we're developing right now, our typical part looks like it's around 6-megahertz. [By comparison, the clock speed of the 6502 is between 1 and 2 megahertz, depending on the computer you use.] at a typical 8- to 12-megahertz.
At what point do you run into problems with increasing the speed?
We see right now a standard off-the-shelf RAM of 6-8 megahertz as the upward limit because of the memory technology. Now, the reader should be familiar with what we're talking about-6 to 8 megahertz-you've heard 4-megahertz, 6-megahertz, 8-megahertz IBM PCs, something like that.
Well, the 65816, when it runs at
the same speed as the Intel processors-meaning the 8086 and the 80286
or the 80386-one of our bus cycles equals four of theirs. That means
that when we talk about 6 megahertz, that's equivalent to the IBM PC
running at 24 megahertz.
What caused you to establish the Western Design Center?
I established the company in May
of 1978, so we're a little more than eight years old now. I founded
the company because I needed to be able to, let's say, express my 1C
design concepts without restriction.
Is that also possible to speed up? Are you working on that?
Yes. What we're doing on that
line is, in the 2-micron technology, we're just now getting first
silicon out. We're just now evaluating the first functional products.
Once we have that evaluated, then we'll know some things that we can
do to modify the process so that we can get higher speed. The other
thing that we're doing is, we're running the same design at
11/2-micron technology, which should yield approximately a two-times
performance increase. So if we're looking at a typical 4-to
6-megahertz now, we'd be looking one-chip application-specific ICs
[integrated circuits] in 1976, because I was designing one. But then
that was dropped.
On the 65C02-did you approach Apple on that?
I never talked to Apple. I never
talked to any personal computer company, believe it or not, when I
developed the 65C02. And I don't know why. It just was the way it
Does that include manufacturing as well as designing?
We have no intention to have a
factory of our own. The reason why we don't want to have a factory of
our own is because we believe the number of factories that are going
to exist in the future is probably limited to 10 in Japan, and
probably 10-20 in the United States, and probably 10 in Europe. So,
we're talking about 30-40 serious semiconductor factories in the
world, and I don't know where you're going to find the money to
compete against those kinds of companies. Therefore, it would be a
bit foolhardy to think that I'm going to raise a half-billion dollars
to get a factory and then fund it with design. I don't think that'll
Where did Apple Computer get your 65C02 for the Apple IIc computer?
They bought directly from my
licensees, GTE and NCR. On the 65816, they [Apple] buy chips from me
for evaluation purposes-we can't talk anything more than that right
now. [At the time of this interview, Apple Computer had not announced
the Apple IIgs, so Mensch was not
permitted to say anything about the company's use of the 65816 chip-Ed.]
Has the cost of chip development changed from your standpoint, or is it pretty much the same?
Well, it's pretty much the same.
I think where we see a lot of activity occurring is in the design
workstations. The design workstations are trying to reduce the cost
of the [chip] development. But it's been my experience that they increase
the cost rather than reduce it because of the cost of the tools. You
have to absorb the cost of the tools-and then what you have are
people needing to be trained in the basic skills of the technology
and also the skills of working with the tools. So it's really a
delaying mechanism rather than a speeding-up mechanism for the first
development. After you've developed a couple of ICs with the new
tools, then you begin picking up speed. But sometimes that's too late.
Why is that?
Well, like I was saying, it slows you down. And so, we didn't have the the expense. It would cost money to do that. We didn't have the resources to buy the latest tools either. At the same time, it's very easy to draw a picture of what you want and then do it. And what you find right now is, the successful companies are still doing it the old way- they work at it. In other words, they use blueprints, they use hand-drawn things; they still use those methods. Now it's changing, but it's not going as fast as the marketing organizations of the workstation companies would like to believe.
What are you currently working on?
One thing I want to mention is that we are in the final stages of development on an applications-specific IC that is meant to save lives. It's going to be implanted in a human being, and it's actually a local area network to be implanted in the human body.
It would serve as a monitoring system?
It's a monitoring and also
stimulus system, meaning that it monitors body functions and it also
stimulates body functions, one of which is obvious- the heart. Other
things are muscles. Other things are hearing assistance devices, and
others will be sight-assistance devices. With this local area
network, we can hang eight processors on it.
What other projects are you working on that grow out of your design efforts?
applications-specific ICs. And we're
using, in the Apple II, low-cost personal computers to develop the
ICs. I'm a visiting professor of computer science at the University
of Strathclyde in Scotland, and we're
looking at putting together a team where we develop the ability to do
the layout graphics for ICs-the graphics editors and also the other
design tools-using an Apple II as a platform.
What does the future hold for you and for the Western Design Center?
Often, I'm asked if I'm going to
sell out to a bigger company. If you look at what I'm trying to do in
my own way, and it could be considered small or big, it depends on
what history will show. But by keeping it in an independent fashion
this way, I believe that I offer more to the general society than if
I was to be gobbled up by some bigger company.
From Compute! Vol. 4 Issue 4 Fall/Winter 1986