bigmessowires
Well-known member
While investigating why Floppy Emu wasn't working on the Mac 128K/512K, I discovered something very interesting in the original 64K ROM used in those machines. It looks like the only reason that external 800K floppy drives didn't work "out of the box" with the 128K/512K was due to a divide by zero bug in some timing code in the old ROM. Were it not for that bug, the 128K/512K could have booted directly from an external 800K drive, instead of the crazy dance of booting from the 400K internal drive with the HD20 init first. And even allowing for the bug, Apple/Sony had an opportunity to work around it with a small design change to the 800K drive, but they didn't.
If anyone's interested, I described the gory details here: http://www.bigmessowires.com/2013/10/17/fixing-30-year-old-apple-rom-bugs/
If anyone's interested, I described the gory details here: http://www.bigmessowires.com/2013/10/17/fixing-30-year-old-apple-rom-bugs/