In February 1985, IBM added two new model types to its 3380 family of direct access storage devices, offering up to a 15 percent increase in performance over the three standard IBM 3380 units. These new extended capability models were available in single capacity (AD4 and BD4) and double capacity (AE4 and BE4) versions. Double capacity models could store up to 5.04 billion characters of information per unit -- twice the storage capacity then available. The new single capacity models could be converted to double capacity at a customer location.
The extended capability models had the same relative arrangement of tracks and cylinders as standard models, allowing customers to expand their current 3380 storage subsystems. (The storage requirements of typical large computer systems in the mid-1980s doubled every two years.)
To accommodate growth in existing installations, the extended capability 3380s were designed to share 3880 storage control units with standard 3380 models. In addition, the extended capability models could be used together in a string of up to four units to meet varying requirements for faster access and for storing larger volumes of data. Four double capacity devices could be connected in a string to store more than 20 billion characters of information. With that configuration, a data processing installation could double its storage capacity without additional floor space, power or air conditioning.
The twofold increase in storage capacity was the result of advances in disk technology and design improvements in the recording heads that wrote information on the disks and retrieved it. The double capacity models had twice as many tracks on each disk "platter." Both new 3380 models had fewer components, which improved availability and reduced maintenance.
The AD4 and BD4 single capacity 3380 models were available at announcement at the same price as the standard AA4 and B04 models -- $88,780 and $64,440 per unit, respectively. The AE4 and BE4 double capacity models were priced at announcement at $134,740 and $110,400, respectively.