In an interview with Keyboard Magazine, David Paich and Steve Porcaro discuss how the keyboard solo in "Rosanna" was created:
David Paich: It's a combination of both of us playing. It's mainly Steve, but we both start at the beginning. When we were doing that song we found we had to put a solo in the middle of it. It was a question of what do we do now. Steve has always said we should use our imaginations. So I kind of started out with trying to incorporate everything we have at our home studio. Instead of doing a regular Wurlitzer or Hammond solo we decided to try different things. We started playing lines and finally sat down and Steve wrote out the initial basis for the solo that you hear now played mostly on the modular system.
Steve Porcaro: It's actually a hilarious story. We thought about how to do that solo for weeks, and it all came together one day at five in the morning.
David Paich: When you hear the opening lines, that's Steve playing a modular trumpet sound. Then we knew we wanted a sequence running down so I programmed a little thing into the Micro-Composer that Steve dropped into the solo. Then there's a Minimoog part, and at the end it's a combination of Steve and I both playing CS-80s, Prophets, a Hammond organ, and a GS1.
Steve Porcaro: There's even a line in there that was from an older solo that David did which I forgot to erase. It was a very pieced-together solo. It was a great example of what can happen when two keyboard players get together and start messing around with technology. You get stuff that you don't plan on getting. Everything was written out and then the written-out stuff was abandoned. It's the right combination of discipline and non-discipline. It's also a good example of what can happen when you're not stuck in a studio with a bunch of people hanging over you saying, "Okay, let's do the keyboard solo now." We did it at home, and I was able to spend a couple of weeks at it. The way it happened wasn't in the norm at all.