Apple SOS

Apple SOS ("Apple Sauce"... hee hee), or "Sophisticated Operating System", was the operating system for the Apple III computer, which was a failure more due to hardware problems than to any problem with its systems software, though its lack of compatibility with the popular Apple II (you could boot in emulation mode, but it didn't share disk files due to different filesystems) may have also been a factor.

The OS was indeed sophisticated for its time; among other things, it used a complex scheme of memory management to allow an 8-bit processor to seamlessly access well over the 64K directly addressable in 8 bits.

The filesystem was kept in the later ProDOS for the Apple II series.

Links

 * Wikipedia article
 * Emulating Apple III