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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. Later, optical disc drives and flash memory units are also classified as DASD.
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 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. IBM direct access storage devices prior to System/360 have a variety of architectures, as do newer devices outside of the S/360 line, but the DASD in IBM mainframe for S/360 to IBM Z use only three DASD architectue
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 and a five byte record identifier (CCHHR).
For devices prior to extended address volumes, the seek address is
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.
IBM refers to the data records programmers work with as logical records, and the format on DASD as blocks or physical records. One block might contain several logical (or user) records or, in some schemes, called spanned records, partial logical records.
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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. Later, optical disc drives and flash memory units are also classified as DASD.
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 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. IBM direct access storage devices prior to System/360 have a variety of architectures, as do newer devices outside of the S/360 line, but the DASD in IBM mainframe for S/360 to IBM Z use only three DASD architectue
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 and a five byte record identifier (CCHHR).
For devices prior to extended address volumes, the seek address is
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.
IBM refers to the data records programmers work with as logical records, and the format on DASD as blocks or physical records. One block might contain several logical (or user) records or, in some schemes, called spanned records, partial logical records.