This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Quite a few DOS 3.3 programs will run fine under ProDOS with no change at all. Unless you have some reason to believe a DOS 3.3 program will not run under ProDOS, it's worthwhile to just copy it to a ProDOS diskette and give it a try. Note: A few 'DOS 3.3' programs, mainly old commercial games, include routines which do direct writes to one or more sectors. This could overwrite parts of a file or a ProDOS directory. It's a good idea to copy risky-looking software to a ProDOS disk you can afford to lose (e.g. a bootable disk with no favorite programs, etc. on it). Boot this disk and try out the program. The main barriers to running DOS 3.3 programs under ProDOS are ... 1. Size: DOS 3.3 leaves more space for programs than ProDOS + BASIC.SYSTEM. Some DOS 3.3 programs are too large for ProDOS. To use them you will need to find ways to reduce program size. 2. Areas of memory used: ProDOS reserves parts of memory, such as Text Page 1 ($0400-$07FF), for various pointers. Some DOS 3.3 programs LOAD or BLOAD into one or more of these areas. For example, some games BLOAD a lo-res pic or Text for you to look at while the rest of the game loads. Normally, ProDOS will refuse to do such loads and the result will be a BUFFERS NOT AVAILABLE error message. To run these programs you will need to unmark the reserved areas which get in the way (see next Q&A) and/or change the location(s) of the program's troublesome LOADs and BLOADs. When ProDOS loads it uses most of the memory in the "language card" areas. Some DOS 3.3 programs also use this memory. If a DOS 3.3 program uses the "language card" it will probably have to be modified in order to run under ProDOS. ProDOS handles buffer allocation differently than DOS. If a DOS 3.3 program lowers HIMEM in order to create space for BLOADing code or data, it may need to be modified to guarantee that the code/area is safe. (See page 237 of Exploring Apple GS/OS and ProDOS 8 or page 7-4 of Beneath Apple ProDOS.) 3. Names: DOS 3.3 file names can be longer and have more kinds of characters than ProDOS names. If a DOS 3.3. program BLOADs, WRITEs, etc. to any files, you will need to be sure that the file names used are compatible with ProDOS. 4. DOS/ProDOS Commands and Syntax: There are, really, very few DOS 3.3 command and syntax differences likely to cause problems under ProDOS. Still, there are some and you will need to check for these and make any indicated mods. 5. Integer BASIC: Int BASIC programs can be transferred to a ProDOS diskette; but, you can not run them directly under ProDOS. If you are willing to move an Int BASIC program to a Text editor, convert the syntax to Applesoft BASIC, and EXEC the resulting Text file, you can obtain an Applesoft BASIC program which you can modify as necessary for ProDOS. 6. It's not DOS 3.3: Very old software, old commercial copy-protected software, and deprotected copies of old software may not be on DOS 3.3 disks. Your first challenge with such software is getting it into a form which will permit transferring files to a ProDOS diskette. When checking and modifying a DOS 3.3 program for ProDOS a good program editor is essential. You will save untold hours of work when you invest in Beagle's tried and true "Program Writer". _________________________ By: Jeff Blakeney