Meet Fractal, an OS made for microarchitecture reverse engineering

Probing how a CPU isolates user code from kernel code is messy work. Researchers patch kernels, write drivers, or boot stripped-down bare-metal programs, and any of those choices change variables they were trying to hold still. Fractal, a new operating system from MIT CSAIL, was built to take that mess out of the loop, and its authors used it to surface previously undocumented behavior in the Apple M1 branch predictor.