Apple Macintosh computers of the 1990s came with a system ROM containing an Open Firmware implementation and the Mac Toolbox required to start the operating system. In many cases this was on a SIMM-like daughter board, and it would have been a true ROM that was unable to be reprogrammed. This is not the end of the story though, and [Doug Brown] set out on the trail of a Flash-based ROM module allowing the firmware on these machines to be updated.
The trail was warm thanks to an Apple developer utility found on a secondhand Mac prototype, allowing ROM flashing. A little disassembly allowed a list of valid IDs to be made, and this info coupled with a bit of reverse engineering from online photos of a real Apple Flash ROM from the ’90s allowed a new board to be created with four Am28F020 chips. He can now flash at will, with such oddities as running ROMs from different machines with the “wrong” startup chime. It’s an interesting little piece of 1990s Mac trivia, settled.
This isn’t the first time we’ve peered at Apple ROMs, indeed some of the older ones had plenty of Easter eggs hidden within.