Software

One of the reasons the Apple II was so successful was that the inner workings of the hardware was made public, whereas other manufacturers treated such things as trade secrets. Another key factor was being one of the first systems for which a disk drive was an expected end-user accessory -- despite a built-in cassette port. Developers took advantage of the Disk II standard, and within a few years there was a veritable explosion of disk software.

The Apple II had two major disk operating systems, DOS 3.x and ProDOS, each of which might be needed to run specific software that didn't automatically boot. DOS 3.1 (not 1.0 due to internal versioning) was released along with the original Disk II. In 1980, DOS 3.3 was the last new version of the original DOS released. It supported increased disk capacities and a new sector format. The new format required a conversion before old disks could be used on the newer disk drive, which had an updated ROM.

_{Mystery House, one of the very first text and graphics adventures, was created in 1980 by On-Line Systems (Sierra) founders Ken and Roberta Williams and released into the public domain in 1987 to celebrate the company's seventh anniversary. While its simple line graphics were visually primitive in comparison to games released just a few years later on the platform, Mystery House established an important precedent.}

Since the original Apple DOS was designed exclusively for Disk II, ProDOS was released in 1984 to make mixed disk formats and hard drives more accessible, as well as faster and more flexible.

Based on the Apple III's versatile Sophisticated Operating System (SOS), ProDOS was able to support the II-series for the entire original run of the systems and through to the present day. In 1986, with the release of ProDOS 16 1.0 on the 16-bit Apple IIgs, the original 8-bit ProDOS software's name was changed to ProDOS 8 with the release of version 1.2. The last version of ProDOS 8, version 1.9, was released in 1990.

_{SSI was a big Apple supporter, with plenty of strategy and role-playing games (such as 1985's Phantasie, shown here) created for the II-series first and then ported to other platforms.}

Dan Bricklin and Bob Frankston created the first "killer app" for the business world with their category-defining VisiCalc spreadsheet software in 1979, and the II-series would dominate the educational market into the 1990s. However, Apple's versatile computers were as good for gaming as productivity and educational programs.