Concurrent DOS 4.1
Released in 1985 by Digital Research Inc.
For IBM PC/XT/AT Computers. 

Disks:
Concurrent PC-DOS 4.1 Startup 1/5
Concurrent PC-DOS 4.1 Utilities #1 2/5
Concurrent PC-DOS 4.1 Utilities #2 3/5
Concurrent PC-DOS 4.1 Applications 4/5
Concurrent PC-DOS 4.1 Reference 5/5

Note: Disk 1 is a 320K CPM-86 formatted disk. 