> The initial versions of the CPU and GPU above were over 200 mm², which is quite large. Conjecture: making them initially a single chip would have resulted in substantially diminished yield. ## Being already at the upper end what can be done A Pentium III (Coppermine) of that time had about 10M transistors and ~100 mm², so a die with more than 200 mm² was most definite at the upper end of what could be done with reasonable expectation of success. Doubling this might have been past what could have done at that time. ## Increasing absolute yield rate Chip fault rate goes not linearly, but exponentially with size. But already when assuming a constant defect rate per wafer, doubling the size will double the fault rate as faults are now be distributed among half the dies. Example: - Let's assume two dies of 250 mm² each or one die of 500 mm² - If a wafer offers 25.000 mm², it gives 100/50 dies (*1) - If the process has an average of 10 defects, then 10 dies are dead (*2) - For the 250 mm² die this gives 90% yield - For the 500 mm² die this gives 80% yield Doing two wafers, one with each, will result in 90+90 good chips enabling the build of 90 machines, while doing the same two with double sized chips gives 40+40 good chips for 80 machines. It's easy to see that doing multiple chips will increase yield and decrease cost per chip. > Was that the reason, or was there another factor? ## Reducing development complexity It saves resources in development to do multiple chips, as that not only reduces complexity a lot but also allows independent schedules for both. This may sound counter-intuitive at first, as they both have to finish before the machine can be built. But it allows each project to advance at their own pace for interim milestones while a common die would need to interlink every step and iteration. A classic problem of complex developments. When looking at development like production, it's much like before: yield is increased by reducing dependence. --- *1 - Simplified by ignoring cut areas and the like. *2 - Simplified by assuming even distribution.