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."IBM coined the term DASD as a shorthand describing disks, magnetic drums, and data cells. Later, optical disc drives are also classified as DASD. The term DASD contrasts with sequential storage media such as magnetic tape, and unit record equipment such as card devices like card readers and punches.

Access methods for DASD include sequential, indexed, and direct. Direct access contrasts with the sequential access method used in tape drives. 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 requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium.

The DASD storage class includes both fixed and removable media.

IBM mainframes access I/O devices including DASD through channels, a type of subordinate mini-processor. Channel programs write to, read from, and control the given device.

The operating system uses a four byte relative track and record (TTR) for some access methods and for others an eight-byte extent-bin-cylinder-track-record block address, or MBBCCHHR, Channel programs address DASD using a six byte seek address (BBCCHH) and a five byte record identifier (CCHHR).

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