Direct-access storage device

A direct-access storage device (DASD) (pronounced /ˈdæzdiː/) is a secondary storage device in which "each physical record has a discrete location and a unique address". The term was coined by IBM to describe devices that allowed random access to data, the main examples being drum memory and hard disk drives.^[1] Later, optical disc drives and flash memory units are also classified as DASD.^[2]^[3]

The term DASD contrasts with sequential access storage device such as a magnetic tape drive, and unit record equipment such as a punched card device. A record on a DASD can be accessed without having to read through intervening records from the current location, whereas reading anything other than the "next" record on tape or deck of cards requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium. Access methods for DASD include sequential, partitioned, indexed, and direct.

The DASD storage class includes both fixed and removable media.

^ IBM Corporation (1975). Introduction to IBM Direct-Access Storage Devices and Organization Methods (PDF). p. 1-1. Archived from the original (PDF) on 2013-12-24. Retrieved 2013-12-23.
^ IBM Corporation (2015). "Serial Direct Access Storage Device Subsystem". IBM AIX V6.1 Documentation. Retrieved December 28, 2015.
^ DellEMC. "Powermax 8000 Joins the Mainframe Solutions Portfolio". Dell EMC Products for Mainframe Environments. Retrieved August 7, 2018.

[Intro-1] IBM Corporation (1975). Introduction to IBM Direct-Access Storage Devices and Organization Methods (PDF). p. 1-1. Archived from the original (PDF) on 2013-12-24. Retrieved 2013-12-23.

[2] IBM Corporation (2015). "Serial Direct Access Storage Device Subsystem". IBM AIX V6.1 Documentation. Retrieved December 28, 2015.

[3] DellEMC. "Powermax 8000 Joins the Mainframe Solutions Portfolio". Dell EMC Products for Mainframe Environments. Retrieved August 7, 2018.

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