PCI bus enumeration, done by brute-force — loop over every bus, device, and function number, write a config address to IO port 0xCF8, read whatever comes back from 0xCFC. All ones means nothing’s there; anything else gets recorded. Once every device on the bus is found, it scans the results for a class code matching AHCI (SATA storage), grabs that device’s BAR5, and starts bringing the controller up — reset bit set, poll until it clears, enable global host control. This is what opens up actual disk read/write instead of relying on BIOS