What is the difference between a modem for a mac and for a pc?

One major difference between Old World ROM Macs and New World ROM Macs, at least in classic Mac OS, is that the Gestalt selector for the machine type is no longer usable; all New World ROM Macs use the same mach ID, 406 decimal, and the actual machine ID is encoded in the "model" and "compatible" properties of the root node of the Open Firmware device tree. The New World ROM also sets the "compatible" property of the root node to "MacRISC2" (machines that can boot classic Mac OS using "Mac OS ROM") or "MacRISC3" (machines that can only boot Mac OS X or another Unix-like system). Also the firmware doesn't include either the toolbox or the 68k emulator, and is so reduced from 4 MB to just 1 MB in size, as these are inside the "Mac OS ROM" file in classic Mac OS and are not needed on other OSes like Mac OS X.