This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Apple II's from an Apple II+ with 64k RAM through the IIgs can run versions of ProDOS up through version 1.9 and run Applesoft BASIC programs under these versions. An older Apple II (with Integer BASIC in ROM) or any Apple II with less than 64k RAM can run ProDOS 1.0 or 1.0.1 but can not use the ProDOS BASIC interpreter (BASIC.SYSTEM). This is not a very useful setup as there are few applications that will work in this environment. Versions 2.x.x of ProDOS require an Apple II equipped with a 65C02, 65C802, or 65C816 such as an Apple IIe-enhanced, //c, IIc+, or IIgs. The current ProDOS is version 2.0.3. By: Rubywand