DOS.MASTER (also: DOS Master) is a program for Apple II computers which allows Apple DOS 3.3 programs to be placed on a hard drive or 3½" floppy disk and run from ProDOS. It was written by Glen Bredon (1932 - 2000) as a commercial program during the late 1980s where it experienced widespread success; it was released into the public domain by his family after the author's death.
DOS.MASTER was created as a result of Apple Computer's abandonment of the DOS 3.3 operating system and its subsequent replacement by ProDOS. Apple provided a program to copy files from DOS 3.3 volumes to new ProDOS volumes; however, programs written for DOS 3.3 did not run on ProDOS volumes. DOS.MASTER enabled a widely installed base of previously ProDOS incompatible programs written for DOS 3.3 to be run under ProDOS. DOS.MASTER took a large ProDOS partition, formatted it as a file, and then created a series of DOS 3.3 volumes within that file. The program allowed the user to create one of four DOS 3.3 volume sizes: 140 KB (the standard capacity of an Apple II 5¼" floppy disk), 160 KB, 200 KB, or 400 KB (the maximum that DOS 3.3 could address). Up to 255 of these volumes could be created on the larger ProDOS partition, space allowing, essentially simulating a very large stack of virtual floppy disk drives.
Modern use
DOS.MASTER is a valuable asset among fans of Apple II retro computing, especially considering the very limited availability of 5¼" floppy disks and floppy disk drives which are required to run DOS 3.3. DOS 3.3 does not support more modern data storage devices like 3½" floppy drives or hard drives. DOS.MASTER can be used with modern computer hardware such as SCSI hard drives and Compact Flash peripheral cards to create multiple DOS 3.3 partitions. Support for DOS.MASTER has even been implemented in modern Apple II archive utilities like faddenSoft's CiderPress.