What makes COS-300 hard to find is that most distributions were on RK05s, DEC's low end 2.5MB front load cartridge hard drive. These were not the most durable of drives and often succumbed to head crashes. However, it was relatively easy to replace the heads, about an hour's work for field service. COS-300 (Commercial Operating System 300) was an OEM package VARs sold to small businesses in the 70's. A typical system was a smaller PDP, an 11/34 or 11/03, RT-11, and the DIBOL compiler, along with something added by the VAR. There were several basic financial packages for the systems, GL/AP/AR/PY or accountant client write-up being typical markets. Your humble author developed some vertical apps for casinos and gas station accounting on a 11/34 COS-300 system. 25 years later I'm still supporting some of that legacy DIBOL code, now migrated to VMS and Alphas. Old business apps never die, they just migrate to the next box... COS-500 was for larger customers, who typically ran RSTS on 10 or more terminals. BASIC was more common on these systems, which could range up to a PDP-11/70. I seem to recall there once was some sort of 5.25" floppy distribution in the early 80's. It had RT-11 v4 and was primarily for 11/23 and 11/73 systems. This was for the odd dual floppy came out with, where two drives shared one spindle and the top floppy was upside down. These were single sided (400KB?) and used FILES-11 for the file system. DMS-500 was DECs answer to the PICK operating system. PICK had made substantial inroads in the medical field, especially hospitals, so DEC came up with the same "put everything in the database" concept. Real CODASYL databases were too big for PDP-11s so DMS-500 was the low end answer if you couldn't afford a PDP-10 or PDP-20. As I understand PICK eventually migrated to the IBM RS/6000 but DMS never got much support from DEC sales. I think there was some version for VMS but it wasn't a major player on the VAX compared to DBMS-32 and later Rdb and Oracle. Jack Peacock