Finding a CPU design bug in the Xbox 360

The recent reveal of Meltdown and Spectre reminded me of the time I found a related design bug in the Xbox 360 CPU - a newly added instruction whose mere existence was dangerous.Back in 2005 I was the Xbox 360 CPU guy. I lived and breathed that chip. I still have a 30-cm CPU wafer on my wall, and a four-foot poster of the CPUis layout. I spent so much time understanding how that CPU's pipelines worked that when I was asked to investigate some impossible crashes I was able to intuit how a design bug must be their cause. But first, some background...