This article describes and differentiates the Mac OS ROM file and the Macintosh bootROM.

Background
Historically, the Macintosh ROM (Read Only Memory) has been structured as one monolithic chunk of code residing on a physical chip on the logic board. It contained the routines needed by the computer at start up time (hardware knowledge, initialization, diagnostics, drivers, and such), as well as quite a bit of higher level Mac OS code.

While a computer needs to have a ROM with hardware-specific code in order to start up, the higher level code was also included in the Macintosh ROM because the Macintosh ROM had its genesis in the original 128K Macintosh computer back in 1983. At that time, ROM was cheaper than RAM, and the available disk space (which was floppy based) was at a premium. This is no longer the case.

The NewWorld architecture, introduced with the original iMac in 1998, separates the hardware-specific and higher level system software into two logically distinct pieces that are software based. Because the ROM information is now software based, it can be updated with software instead of requiring a hardware service.

bootROM vs. Mac OS ROM

In the NewWorld model, the bootROM holds most of the hardware-specific components needed to boot the specific logic board. Low-level hardware-specific code still exists in firmware (ROM) in order to handle the computer's start-up activities. This code fits into one physical ROM chip called the bootROM. The bootROM has the hardware-specific code and description of the hardware needed to start up the computer and provide common hardware access services the operating system might require.

The Mac OS ROM image file, present in the system folder, contains boot-time Mac OS routines and hardware-specific software components that are common to many Macintosh computers.

Both the bootROM and the Mac OS ROM can be updated, if need be, to provide enhanced functionality or to fix a known issue.

Determining whether to update your bootROM or Mac OS ROM

Before updating either the bootROM or the Mac OS ROM file, you should check the version you currently have installed. Apple System Profiler is the easiest method to determine the version of the Mac OS ROM and the bootROM. Simply click the triangle next to "Production information" under the System Profile tab. The information will look like this:

Look for the Boot ROM version and the Mac OS ROM file version and compare it with the update to ensure you are updating to a later version.

What about Open Firmware?

Open Firmware is the non-proprietary name of firmware complying with IEEE Standard 1275-1994. OpenBoot is Sun Microsystems trademark for the firmware product shipping on over one million SparcStations and SPARCServers since 1989.

Among Open Firmware's many features, it provides a machine independent device interface, which can be used to boot plug-in cards without providing OS-specific and/or machine dependent binary programs on the plug-in card. This feature enables plug-in card manufacturers to easily support several independent computer architectures without needing to supply different firmware for each one.

More information

Additional information on the Mac OS ROM, bootROM, and Open Firmware can be found on the Apple Developer Connection Web site at http://www.apple.com/developer . A search mechanism is available to assist you in finding the information you require.