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A Historical Look at the VAX: Microprocessor Economics

January 24, 2006 by John Mashey

Pages: 1 2 3 4 5

Editor's Note: This series of articles first appeared last summer in
the comp.arch newsgroup, in a series of posts by John Mashey. Since
then, it has been updated, edited and enriched with additional
material and graphs by David Kanter, all with permission of the
author.

Introduction

The VAX was an orthogonal, 32-bit CISC instruction set architecture
and an associated family of CPUs and systems, produced by the Digital
Equipment Corporation. The VAX is considered to be the quintessential
CISC architecture and VAX systems (often called VAXen) typically used
an operating system known as VMS and were shipped from 1977 till
1992, when the line was discontinued in favor the Alpha. In the wake
of the death of the Alpha architecture, there have been many
discussions regarding the feasibility of cheap, fast and timely VAXen
in the early 1990's. Many have asserted that if DEC had continued
with the VAX, rather than transitioning to the Alpha that the company
and its products would be in better shape today. Unfortunately, most
of this discussion is filled with fantasies about what could have
been; the issue is not just what was technically feasible, but what
was cost-competitive and time-to-market competitive.  

This series of articles will examine the issues confronting DEC and
the VAX architecture in the early 1990's. The first part covers some
of the economics of systems and microprocessors. The second part in
this series will address the competitive challenges that DEC faced
around the time of the Alpha. The third and last part will compare
the instruction set difficulties facing out-of-order VAX, x86 and s/
360 microprocessors and bring everything together.

The Design Cost of Microprocessors

Suppose it costs $50,000,000 to design a chip and get into to
production.  The amortized cost per chip for engineering versus total
unit volume is shown below in Figure 1.

[ooo-vax-1]
Figure 1 - Hypothetical Development Costs per CPU

Alternatively, if you can sell 5,000,000 chips, then you could spend
$500,000,000 on development, and still only have an engineering cost
per chip of $100. It is easy to see that as a result of these
economics, high volume designs achieve high performance with relative
ease. Additionally, higher production at a fab increases the quality
of silicon, further improving the performance of a high volume
design.

To put this in perspective, I have included the MicroVAX-II and IBM
PC volumes over the periods 1985-1987. Under this hypothetical model
that assumes equal development costs, the difference in per unit R&D
costs is roughly two and a half orders of magnitude.


Pages:    1 2 3 4 5  Next >>

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