IBM 355 Disk Storage

The addition of disk storage to the IBM 650 Magnetic Drum Data Processing Machine made possible "single step processing." Instead of accumulating data to be processed in stages, transactions could now be processed randomly as they occurred and every record affected by the transaction could be automatically adjusted in the same processing step. Each IBM 355 held 50 disks subdivided on each side into tracks for the storage of almost all active accounting records. Up to four IBM 355 units could be connected to the 650 system.