Boot Sequence
When powered on, the Apple III ran through system diagnostics, then read block number one (zero-indexed) from the built-in diskette drive into memory and executed it. SOS-formatted diskettes placed a loader program in block one. That loader program searched the diskette directory for a file named SOS.KERNEL, the kernel and API of the operating system. The SOS loader program loaded and executed the SOS.KERNEL file, which in turn searched for and loaded a file named SOS.INTERP (the interpreter, or program, to run) and SOS.DRIVER, the set of device drivers to use. Once all files were loaded, control was passed to the SOS.INTERP program.
Since Apple ProDOS uses the same file system as SOS, and since ProDOS stores its own boot code in block number zero rather than block number one, SOS and ProDOS can co-exist on the same medium. Some software, e.g. 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.
Read more about this topic: Apple SOS
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