Perry Hung @ MIT made this great Linux boot sequence visualization, he writes-
This is a visualization of a linux boot sequence where each function is a node and each edge represents a function call, direct branch, or indirect branch. Nodes are laid out using an unweighted force-directed layout algorithm, where each node is simulated as if it were electrically repulsive and had springs between nodes.
The little “lobe” on the left is made up the interrupt processing routines (irq vectors, irq_svc, etc). The tail at the top is the bootloader. The main thing in the middle is the linux boot sequence.
The entire graph represents a call chain from the bootloader up until it jumps into userspace to a shell prompt…