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Starting the Operating System

After constructing the device tree in system RAM, the Power Macintosh startup firmware selects some or all of the following startup devices, based on an order of priority stored in the system hardware and on the presence of suitable device properties in the device tree:

a keyboard (or other input device)
a display (or other output device)
a boot device (mass storage or ROM, indicated by the boot path environment variable) that contains operating-system code

The Open Firmware code normally loads the operating system into memory and starts it using the Forth go command. In the case of Mac OS, Open Firmware transfers processor control to the Mac OS ROM, which begins the Mac OS startup process. If the Open Firmware user interface is invoked, however, the Open Firmware code will continuously poll the input device for characters and write output characters to the display, using the FCode drivers previously installed. This can let the user choose an operating system or perform other OS-independent configuration tasks. For further details, see Open Firmware User Interface.

For further details of the normal Macintosh startup sequence, see Chapter 10 of Technical Introduction to the Macintosh Family, described in Supplementary Documents .