In Intel jargon, simultaneously developing multiple designs for the same manufacturing process is known as the 'tick tock' model. One of the reasons for applying this model is that the behaviour of small transistors is uncertain to some degree. When work on Penryn and Nehalem commenced in May 2004, the most advanced manufacturing technology available was 90nm. Back then, it was hard to say how 45nm circuits would behave, just as it is now difficult to make predictions on how things will go at 22nm. There are always rough estimates to work from, but these change as more decisions are made and theory is put into practice.
Changing characteristics of a manufacturing process, also known as 'design rules', present significant challenges to designers. If problems are tackled in too radical a fashion, ideas may turn out to be impossible to implement. On the other hand, doing things conservatively may lead to suboptimal use of the possibilities that the factories, which cost billions, have to offer.
This is why Intel chooses to explore two different approaches simultaneously, with an offset of about a year. For the 45nm generation, Penryn represents the conservative approach while Nehalem is the progressive one. Every obstacle that the first team encountered was a lesson for the second one, which then had an extra six to twelve months to solve it. This luxurious position enabled the second team to concentrate much more on new features, such as the integration of multithreading, a memory controller and a video chip.
Intel plans to follow this strategy in the coming years, meaning that the 32nm generation also has two entries on the roadmap: 'Westmere' will be a conservatively shrunk version of Nehalem, while the progressive 'Sandy Bridge' is to add a good deal of new features. And things don't stop there: Fisher told us that he is already working on a 22nm design, and elsewhere within Intel, teams are already figuring out what features might be added into the 16nm generation.