drum, then rotated once to read or record a diagonal track. Then drive then wound the tape a small step forward and the head rotated to do the next track. Depending on technical definitions this might be even considered a first example of a digital helical scan recording, long before Exabyte's helical drive (which was based on analog video helical recording systems developed earlier). When free disk space was required a group of cylinders were selected to be destaged to tape, these were transferred with minimal or no change of format. Each tape could store 202 cylinder images of 19 tracks each, half of the 404 cylinders in a 3330 disk pack. Cylinder locations on the tape were fixed and identified by markers along the edge. Models Several models of the 3851 were available. The smallest A1 holding 706 cartridges storing 35.3GB, while the largest A4 held 4,720 cartridges storing 236GB in a long unit. All of the units were also available in the "B models" which added a second controller for on-line backups, as well as offline storage. A second series was released on March 6, 1980, doubling maximum capacity to 472GB. The entire series was discontinued on August 5, 1986. Notes References External links Columbia University's MSS IBM Archives IBM details of tape recording method IBM description and announcement Good photos of tape cartridge Frankfurt museum's MSS (with video, in German) 3850 Computer storage tape media