I have a legal collection Mac ROM images (i.e., I own each respective Mac.). Amongst this collection are some copies. As many of us know, each ROM starts with a 32-bit checksum. I have one specific pair of ROMs from the same model of Mac, that start with the same checksum, but they are not identical. In fact they are VERY different.

So I played with "sum" and "cksum" on my OS X machine, experimenting with things like zeroing the stored checksum, etc. But I was not able to get any ROM to sum out to the checksum. Does anybody have any idea of exactly how this checksum is computed? Even a general description if anybody knows how this works would do because I could write a quick dirty app to check it.