The Apple II disk bootstrap process looks at the first byte of track zero sector zero to see how many sectors long the bootstrap code will be. It then loads these sectors at location 0x0800 and jumps to address 0x0801, just after the "number of boot sectors" byte.
Normally the number of boot sectors is 1, but some custom boot loaders use more. I've got a few disk images which say 5.
So when there are multiple boot sectors, are they the first physical sectors, disregarding any sector interleave? Or should the interleave for the logical sectors of DOS 3.3 be used? (Surely ProDOS interleave would never be a factor?)
When dealing with disk image files, the common .DSK format (aka .do and .po) have no header tell which sector ordering to use. They use either DOS 3.3 order or ProDOS order. They never use physical order. For OS disks you can look for OS structures at known offsets on the disk to determine the sector ordering. But some games don't use any OS disk structures.
Which sectors did the Apple 2 load, and what should emulators and disk image utilities do?
I've found some annotated disassembly of the boot process with commentary on Usenet
comp.sys.apple2 from four years ago. So far it's difficult for me to understand not being an Apple II guy. I'll keep trying to grok it while waiting for somebody who knows this stuff to come along...