DOS 3.3 was the final revision of the original Disk Operating System for the Apple II line of computers. DOS 3.3 was very simple; directories were not supported, so all your files were in one big directory on each of your volumes. DOS 3.3 could only support 140K 5.25" floppy disks natively, though enterprising individuals did sometimes hack it to support other devices.

The primary advantage that DOS 3.3 had for Apple II systems was that it was small. Very small. For this reason, it was frequently used on commercial game disks, even well after ProDOS, DOS 3.3's successor, came about. DOS 3.3 was also very easily hackable, providing for copy protection, improvement in disk performance, and much more.

The DOS filesystem was typed; each file had a filetype associated with it:

  • B - Used for Binary files
  • A - Applesoft BASIC program
  • I - Integer BASIC program
  • T - A "text" file, also used for EXECutable scripts
File size was measured in 256 byte sectors, so rather than say "That's a 3K file", you'd say "That's a 12 sector file".

One feature that DOS 3.3 had that ProDOS did not was that you could actually format disks (GASP!) from the command prompt, rather than have to run special disk utilities. You would format a disk with the INIT command, which would also designate a program to be automatically run on boot, known as the "HELLO" program.