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 developed DASDs for use with mainframe computers and some minicomputers. Disk drives, magnetic drums, data cells and optical disc drives are all classified as DASDs.

Access methods for DASDs 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 DASDs 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).

When the 2321 data cell was discontinued in January 1975, the addressing scheme and the device itself was referred to as CHR or CTR for cylinder-track-record, as the bin number was always 0.

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