62 COMPUTER ENGINEERING
d. Compound Cost. The costs are taken altogether. In terms of a sub state- of-the-art product, the costs are compound.
3. Manufacturing learning. Learning curves and forgetting curves really matter. Left alone, a typical product may go down three alternative paths (Figure 27):
a. c = b X 0.95t (a decrease of 5 percent/year)
b. c = b (staying constant with little attention)
c. c = b X l.06t (increasing with inflation as little learning occurs after many units are produced)
Where c = cost at time, t (in years), and b = base cost.
Mid-Life Kicker for Product Rejuvenation
By enhancing an existing product (the "mid-life kicker"), one can improve the cost/performance metric of a given product. This is non-trivial, and for certain products must be inherent (i.e., designed in). Under these conditions, improvements in cost go immediately to get the product back onto the state-of- the-art line. For example, a factor of 2 in performance halves cost/performance. The effect
Figure 27. Product cost versus time within manufacturing learning.
of doubling the density of a disk is to move the product back to the state-of-the-art line by a time shift. The preceding formula gives:
dt = 4.45 X 1n (0.5) = 3.1 years
This situation is shown in Figure 28 and is com pared with a 5 percent per year learning curve.
The discussions above have attempted to show how technology progress, particularly in the areas of semiconductor logic, semiconductor memories, and magnetic memory media, have influenced progress in the computer industry and have provided choice and challenge for computer design engineers.
As was implied in the Structural Levels-of- Integration and Packaging Levels-of-Integration Views of Chapter 1, computer engineering is not a one-dimensional undertaking and is not simply a matter of taking last year's circuit schematics and this year's semi conductor vendor catalogues and turning some kind of design process crank. Instead, it is much more complicated and includes many more dimensions.
Two additional dimensions with which a discussion of computer engineering must deal, be fore going on the DEC computers as case studies, are packaging and manufacturing. These are discussed in Chapter 3.
Figure 28. Product cost improvement by enhancement of cost/function.