A direct-access storage device 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 hard disk drives, magnetic drums, and data cells. Later, optical disc drives and flash memory units 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.
Architecture
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.
CTR (CHR)
The operating system uses a four byte relative track and record 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.
M represents the extent number within the allocation
BB representing the Bin,
CC representing the Cylinder,
HH representing the Head, and
R representing the Record number.
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 disk as blocks or physical records. One block might contain several logical records or, in some schemes, called spanned records, partial logical records. Physical records can have any size up to the limit of a track, but some devices have a track overflow feature that allows breaking a large block into track-size segments within the same cylinder. The queued access methods, such as QSAM, are responsible for blocking and deblocking logical records as they are written to or read from external media. The basic access methods, such as BSAM, require the user program to do it.
CKD
CKD is an acronym for Count Key Data, the physical layout of a block on a DASD device, and should not be confused with CTR, which is the block address used by the channel program.
FBA
In the 1970s, IBM introduced fixed block architecture for mainframes. At the programming level, these devices do not use the traditional CHR addressing, but reference fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remains unaware of the underlying storage arrangement, which stores the data in fixed physical block lengths of 512, 1024, 2048, or 4096, depending on the device type. For some applications, FBA not only offers simplicity, but an increase in throughput. FBA is supported by VM/370 and DOS/VSE, but not MVS or successor operating systems in the OS/360 line.
Access
The programming interface macros and routines are collectively referred to as DAM: direct access methods.
IBM in its 1964 first version of the "IBM System/360 System Summary" used the term File to collectively described devices now called DASD. Files provided "random access storage'" At the same time IBM's product reference manual described such devices as "direct access storage devices" without any acronym. An early public use of the acronym DASD is in IBM's March 1966 manual, "Data File Handbook." The earliest non-IBM use of the acronym DASD found by the "Google ngram viewer" to refer to storage devices dates from 1968. From then on use of the term grew exponentially. Both drums and data cells have disappeared as products, so DASD remains as a synonym of disk, flash and optical devices. Modern DASD used in mainframes only very rarely consist of single disk-drives. Most commonly "DASD" means large disk arrays utilizing RAID schemes. Current devices emulate CKD on FBA hardware.
