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The system software in the new iMac family of computers uses the ROM-in-RAM approach first used in the original iMac computer. With this approach, a small ROM contains the code needed to initialize the hardware and load an operating system. The rest of the system code that formerly resided in the Mac OS ROM is loaded into RAM from disk or from the network.
The small ROM that is needed for the computer's start-up activities, called the boot ROM, is only 1 MB in size. It includes the hardware specific code and tables needed to start up the computer, to run Open Firmware, to provide common hardware access services, and to load the Mac OS ROM image.
High level software resides in an image called the Mac OS ROM that is read into RAM before the Mac OS begins operation. Once the Mac OS begins operation, the Mac OS ROM image in RAM behaves in the same way that the corresponding code in ROM formerly did. Most of the changes are completely transparent to the Mac OS.