Editing Apple SOS (section)

===Boot sequence===
[[File:Sos.svg|thumb|right|Structure]]
When powered on, the Apple III runs through system diagnostics, then reads block number zero from the built-in [[floppy disk|diskette]] drive into memory and executes it. SOS-formatted diskettes place a [[bootloader|loader]] program in block zero. That loader program searches for, loads, and executes a file named SOS.KERNEL, which is the [[Kernel (operating system)|kernel]] and [[API]] of the operating system. The kernel, in turn, searches for and loads a file named SOS.INTERP (the interpreter, or program, to run) and SOS.DRIVER, the set of [[device driver]]s to use. Once all files are loaded, control is passed to the SOS.INTERP program.<ref name="jeppson">{{cite journal| title=John Jeppson's Guided Tour of Highway III|url=https://archive.org/details/softalkv3n09may1983/page/100|first=John|last=Jeppson|journal=Softalk|volume=3|issue=9|date=May 1983|pages=100–112|publisher=Softalk Publishing|issn=0274-9629|oclc=6729571}}</ref>

[[Apple ProDOS]] uses the same [[file system]] as SOS. On a disk formatted by ProDOS, the ProDOS loader and SOS loader are written to blocks zero and one, respectively. The ProDOS loader includes code that can execute on an Apple III, and which will [[chain loading|chainload]] the SOS loader from block one, so SOS and ProDOS can co-exist on the same volume. Some software, such as [[ADTPro]], makes use of this to store Apple II and Apple III versions of a program on the same disk, which is then bootable on both systems.