Apple II DOS
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Apple II DOS was used on the Apple II series of computers (Apple II, II+, IIe, IIc) until it was superseded by the later ProDOS. It came with the Disk II floppy disk drives released in 1978. Prior to this, Apple computers were used with audio cassettes as their data storage medium and their only "operating system" was the built in monitor/mini-assembler for machine/assembly-language operations, and Integer BASIC for BASIC programming.
There were no released versions 1.x or 2.x, since those numbers were used in internal development for pre-release versions. 3.0 was prepared as the intended first public release, but was replaced by a version with some more changes labeled 3.1 which became the first Apple DOS, but that in turn didn't last long before being replaced with 3.2, which became the first Apple DOS that many users remember. That version used 5 1/4" Apple II 13 sector disks, with the Apple DOS file system. A maintenance release with minor fixes and updates called 3.2.1 followed.
Later, Apple DOS 3.3 came out, using the Apple II 16 sector disk format, which required a ROM upgrade to disk drives to support it. This upgrade caused old-format disks no longer to work, but a utility called MUFFIN could read files from old disks and port them over to new ones.
The later ProDOS operating system would support the newer 3 1/2" disk formats.