This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
DOS 3.3 is the first DOS to be widely used on Apple II computers. Many programs were written to use DOS 3.3 commands and saved on DOS 3.3 diskettes. Apple 'officially' replaced DOS 3.3 with ProDOS back in the early '80's. However, DOS 3.3 continues to be popular with II users. To get DOS 3.3 from the net, see Csa21MAIN4: Get It- Links to popular software packages. Another Apple II DOS is the one introduced with Apple Pascal. Compared to DOS 3.3 or ProDOS, the Apple Pascal DOS is a very limited, cumbersome operating system. -------------------