Open Boot is a portable boot firmware system. Boot firmware is the ROM-based software
that controls a computer from the time that it is turned on until the primary operating system has taken control of the machine. The main job of boot firmware is to initialize the hardware and then to “boot” (load and execute) the primary operating system. Secondary jobs include testing the hardware, managing hardware configuration information. and providing tools for hardware and software debugging.
Open Boot is portable in the sense that its design is not tied to any particular processor family nor to any particular expansion bus. Open Boot was specifically designed to support a variety of different processor Instruction Set Architectures (ISAs) and different buses.

ESC_1992_Vol1_Page65_Bradey – The P1275 Open Boot Standard for Boot Firmware.pdf