When the power to a Macintosh computer is turned on, the BootROM firmware is activated. BootROM (which is part of the computer’s hardware) has two primary responsibilities: to initialize system hardware and to select an operating system to boot. BootROM has two components to help it carry out these functions:
POST (Power-On Self Test) initializes some hardware interfaces and verifies that sufficient RAM is available and is usable.
Open Firmware initializes the rest of the hardware, builds the initial device tree (a hierarchical representation of devices associated with the computer), and selects the operating system to use.