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Original Power Mac G4 Developer Note
Boot ROM
The boot ROM contains the Open Firmware boot code.
To minimize the number of pins on Uni-N, the boot ROM is connected to the 66 MHz PCI bus. The boot ROM uses the thirty two PCI AD lines and four PCI byte enable lines for address and data. Uni-N has separate pins for Chip Enable, Output Enable, and Write Enable signals to keep the ROM from interfering with proper PCI bus operation.
The boot ROM consists of 1 MB of on-board flash EPROM. The boot ROM includes the hardware-specific code and tables needed to start up the computer, to load an operating system, and to provide common hardware access services.
The components that comprise what is referred to as the Mac OS Toolbox are no longer in the on-board firmware ROM. The Mac OS Toolbox image file, called Mac OS ROM, is copied from the boot device (hard disk or CD-ROM) into RAM before the Mac OS begins operation. For more information about this ROM-in-RAM approach, see
ROM in RAM
.
© 2000 Apple Computer, Inc. (Last Updated 13 Jan 00)