IBM drum storage

Lore & Background

IBM drum storage was used as main memory in computers such as the IBM 305 and IBM 650, and as secondary storage for the 700/7000 series and System/360 series. Specific models include the IBM 731 for the IBM 701, the IBM 732 for the IBM 702, the IBM 733 for the IBM 704 and 709, and the IBM 734 for the IBM 705. The IBM 7320, announced on December 10, 1962, was a vertically mounted head-per-track device with 449 tracks, used on the IBM 7090 and 7094 and later on early System/360 systems as a count key data device. It attached to 709x systems via an IBM 7909 Data Channel and IBM 7631 File Control unit, and to System/360 via a channel and IBM 2841 Storage Control unit. The 7320 was superseded by the IBM 2301 in mid-1966. The IBM 2301, introduced in the late 1960s, was a vertically mounted drum rotating at about 3,500 rpm with head-per-track access, 800 physical tracks grouped into 200 logical tracks of 20,483 bytes each, and a capacity of 4 MB. It attached to System/360 via a selector channel and IBM 2820 Storage Control unit. The IBM 2303 shared the same physical specifications as the 2301 but read and wrote one physical track at a time, reducing its data transfer rate.

Reader's Guide

IBM drum storage played a significant role in early computing by providing random access storage at a time when magnetic tape and punched cards were common. Drums were used as main memory in systems like the IBM 305 and IBM 650, and as secondary storage in the 700/7000 series and System/360 series. The IBM 7320, with its head-per-track design and 8.6 ms average rotational delay, offered faster access than earlier drum devices. It was retained for early System/360 systems as a count key data device before being superseded by the IBM 2301 in mid-1966. The IBM 2301, with a capacity of 4 MB and a data transfer rate of 1,200,000 bytes per second, represented a later generation of drum storage for high-end System/360 models. The IBM 2303, a variant with a lower transfer rate, provided an alternative for systems using the IBM 2841 Storage Control unit. These drum devices were eventually largely replaced by hard disk drives, but they were a key step in the evolution of direct-access storage.

Did You Know?

• The IBM 7320 had 449 tracks, including 400 data tracks, 40 alternate tracks, and 9 clock/format tracks.
• The IBM 2301 had 800 physical tracks grouped into 200 logical tracks, each holding 20,483 bytes.
• The IBM 7320 was announced on December 10, 1962, and discontinued in 1965.
• The IBM 2303 had the same physical specifications as the 2301 but a data transfer rate of 312,500 bytes per second.