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Direct Access Storage Device

A direct access storage device, or DASD, is a secondary storage device in mainframe computers that has low access time for all of its capacity. The term DASD was introduced by IBM to describe disk drives, magnetic drums, and data cells. The direct access capability of those devices was quite different than that of sequential access used in tape drives because it required a substantially long time to access a distant point in a medium. IBM computers access devices through channels, which are subordinate mini-processors. Channel programs read from, write to, and control the given device. They also address data through a process called module-bin-cyl-trk-rec, which is an eight byte address divided into 16 bit-components representing the module and bin, cylinder, track, and the record number. Once the data cell was discontinued, the process as a whole and the device itself was referred to as the cylinder-track-record. IBM labeled the data that programmers worked with as logical records, and how they were stored on disc as physical records. One physical record could contain several logical records. Physical records could have any size that the cylinder could hold, although usually they did not exceed the capacity of a single track. In the 1970s, IBM introduced fixed block architecture, which were devices that referenced fixed-length physical records by numbers instead of using the traditional CHR addressing. The application programmer still remained unaware of the underlying storage arrangement, which stored the data in fixed physical lengths of 512, 1024, 2048, or 4096. For many applications, FBA also brought out an increase in throughput. Today, drums and data cells have disappeared as products, so DASD remains as a synonym of a disk device. Modern DASD used in computers very rarely consist of single disk-drives: most commonly “DASD” means large disk arrays utilizing RAID schemes.



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