Booting Mac Plus right from ROM + some games! — #50

Post #50 by trag

Source Forum	68kMLA
Category	Troubleshooting
Post Date	Thu, 4 Dec 2014 - 23:14
Original URL	https://68kmla.org/bb/threads/booting-mac-plus-right-from-rom-some-games.572/

Post

bigmessowires said:
Cool. The same hook for "test software" appears several places in the Plus ROM (128K ROM) too. It's literally the very first thing the ROM does when booting. If the long value at $F80000 is $55AAAA55, then it jumps to the address pointed to by whatever's at $F80004. There are three or four other similar checks during the boot sequences, at other addresses in the $F800xx range. I wonder what you'd do with that. I wonder how you could even add extra ROM that would get activated by memory accesses to that address range.

I think this would be most useful for some kind of temporary clip-on testing device containing extra software to do validation testing at the factory. If you want to permanently change the Mac's ROM-based behaviors, you're probably better off just replacing the ROMs.

I think you would do this with some additional hardware and/or replacement ROMs. It's gives a built-in method to add functionality. I agree that it probably makes the most sense that Apple had it in there for some kind of clip on test hardware or similar. But as long as it's there, we can bend it's true purpose to our ends, yes?

In this particular instance, I was wondering if Rob could have used this hook for loading his code, instead of replacing the .print driver as he did.