Upon startup, a computer requires hardware-specific Read Only Memory in order to operate. The Mac OS ROM, formerly located on a chip on the logic board, contains a significant amount of higher-level Mac OS code, as well as the routines the Mac uses when booted (including initialization, diagnostics, and drivers).

The reason for having all this higher-level code on the logic board dates back to 1983. In the days of the original 128k Macintosh, ROM was much cheaper than RAM and using actual disk space for the boot ROM would have been wasteful. Now that these problems no longer exist, Apple has introduced its so-called "NewWorld architecture", which made its first appearance on the original iMac in 1998. On NewWorld machines, the information which had been stored in the Mac OS ROM is now split up into two distinct components: the bootROM and the Mac OS ROM.

The bootROM lives in firmware on a ROM chip on the logic board. It contains the very low-level hardware-specific code needed for startup and common hardware interactions.

The Mac OS ROM, containing slightly higher level code for starup routines, lives on as a file in the System Folder. Unlike the old Mac OS ROM, both the bootROM and the Mac OS ROM can be updated with ease by software.

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