Direct access storage device - AbsoluteAstronomy.com

Mainframe computer

Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

s and some minicomputer

Minicomputer

A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...

s, a direct access storage device, or DASD (icon), is any secondary storage device which has relatively low access time relative to its capacity.

Historically, IBM

IBM

International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

introduced the term to cover three different device types:

disk drives
magnetic drums
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....
data cells
IBM 2321 Data Cell
The IBM 2321 Data Cell announced in April 1964 was a direct access storage device for the IBM System/360. It could hold up to 400 million bytes of data, but its access time was approximately 450 milliseconds.-Characteristics:...

The direct access capability, occasionally and incorrectly called random access
Random access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...

(although that term survives when referring to memory or RAM), of those devices stood in contrast to sequential access used in tape

Magnetic tape data storage

Magnetic tape data storage uses digital recording on to magnetic tape to store digital information. Modern magnetic tape is most commonly packaged in cartridges and cassettes. The device that performs actual writing or reading of data is a tape drive...

drive

Tape drive

A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...

s. The latter required a proportionally long time to access a distant point in a medium.

Architecture

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

CTR (CHR)

Channel programs address data through a scheme called module-bin-cyl-trk-rec or MBBCCHHRR, an eight byte address divided into 16 bit-components representing the module and bin (for data cells), cylinder (for discs), head (or track), and the record number. When the 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 referred to the data records programmers worked with as logical records, and how they were stored on disc as blocks or physical records. One block could contain several logical (or user) records or, in some schemes, partial logical records.

Physical records could have any size up to the limit of a cylinder, although in usual practice, blocks or physical records did not exceed the capacity of a single track.

CKD

CHR/CTR acronyms should not be confused with CKD

Count Key Data

Count Key Data is a disk data architecture. Each physical disk record consists of a count field, an optional key field, and a data field with error correction/detection information appended to each field and gaps separating each field...

, which refers to Count Key Data

Count Key Data

, the layout of an addressable data record on a CTR disc.

FBA

In the 1970s, IBM introduced fixed block architecture

Fixed Block Architecture

Fixed Block Architecture is a disk layout in which each addressable record on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, termed logical block addressing and usually having a constant addressable block size of 512 bytes.- Count Key...

, or FBA. At the programming level, these devices did not use the traditional CHR addressing, but referenced fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remained unaware of the underlying storage arrangement, which stored the data in fixed physical block lengths of 512, 1024, 2048, or 4096.

For many applications, FBA not only offered simplicity, but an increase in throughput. GOAL Systems of Columbus, Ohio, discovered that an FBA emulator

Emulator

In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...

written for VM by Bill Jurist delivered an unexpected boost of speed.

Access

The programming interface macros and routines were collectively called DAM: direct access methods.

DOS/VSE

DAmod/DTFDA – direct access
SDmod/DTFSD – sequential disc
ISmod/DTFIS - indexed sequential
VSAM – Virtual Storage Access Method

MVS, OS/390

BSAM
Basic sequential access method
In IBM mainframe operating systems, Basic sequential access method is an access method to read and write datasets sequentially. BSAM is available on OS/360, OS/VS2, MVS, z/OS, and related high-end operating systems....

- Basic Sequential Access Method
QSAM - Queued Sequential Access Method
BPAM - Basic Partitioned Access Method
BDAM - Basic Direct Access Method
VSAM – Virtual Storage Access Method

Present terminology

Both drums and data cells have disappeared as products, so DASD remains as a synonym of a disk device. Modern DASD used in mainframes only very rarely consist of single disk-drives: most commonly "DASD" means large disk array

Disk array

A disk array is a disk storage system which contains multiple disk drives. It is differentiated from a disk enclosure, in that an array has cache memory and advanced functionality, like RAID and virtualization.Components of a typical disk array include:...

s utilizing RAID schemes.