Direct-access storage device

Direct-access storage device (DASD)
Company type	Hardware
Industry	Computer storage
Predecessor	Drum memory, hard disk drives
Founded	1960s
Founder	IBM
Successor	Optical disc drives, flash memory devices
Area served	Computer mainframe systems
Products	Secondary storage devices with random access capability
Owner	IBM (originator)
Parent	IBM

A direct-access storage device (DASD)—pronounced —refers to a type of secondary storage hardware where each individual record is assigned a distinct address. The term was originally introduced by IBM and describes devices that provide random access capability to data, initially encompassing drum memory and hard disk drives. Over time, the DASD category expanded to include optical disc drives and flash memory devices.

DASD differs fundamentally from sequential access storage such as tape drives and unit record equipment like punched card systems. Unlike sequential media, DASD allows direct retrieval of any record without the need to process preceding records, thereby enhancing data access speeds. It supports various access methods including sequential, partitioned, ISAM, and direct. Both fixed and removable storage formats fall under this classification.

Architecture

IBM mainframe systems interact with I/O peripherals, including DASD, through specialized processors called channels. These channels execute Channel programs that manage reading, writing, and controlling the devices.

CTR (CHR)

Certain access methods utilize a four-byte relative track and record (TTR) or an eight-byte extent-bin-cylinder-track-record address (MBBCCHHR). Channel programs communicate with DASD using a six-byte seek address (BBCCHH) combined with a five-byte record identifier (CCHHR).

M = extent number within the allocated space
BB = bin number (applicable to IBM 2321 data cells)
CC = cylinder number
HH = head or track number
R = record or block number

Following the phase-out of the 2321 data cell in 1975, addressing conventions transitioned to CHR or CTR formats with the bin field consistently set to zero.

IBM categorizes data as logical records, which are stored on DASD as blocks or physical records. A single block may contain multiple logical records or fragments of spanned records. Physical record sizes vary but do not exceed a single track’s capacity; some devices permit track overflow to divide large blocks within the same cylinder.

Access methods such as QSAM automate blocking and deblocking of logical records during I/O processes, whereas simpler methods like BSAM place this responsibility on the application software.

CKD

The term CKD stands for Count Key Data, describing the physical format of a block on DASD media. This is distinct from BBCCH and CCHHR addressing schemes used by channel programs. The acronym CTR may denote either address format depending on the channel command issued.

FBA

Introduced by IBM in 1979, Fixed-block architecture (FBA) was designed for mainframe environments. Unlike conventional CHR addressing, FBA accesses fixed-length blocks by numerical index, resembling sector-based addressing found in minicomputers. Block sizes are standardized at 512, 1024, 2048, or 4096 bytes depending on the device model. IBM also developed new channel commands enabling asynchronous operations similar to those used with ECKD.

FBA provides a streamlined design and can improve throughput for certain workloads.

Support for FBA exists in VM/370 and DOS/VSE, but it is not compatible with MVS or subsequent OS/360 derivatives.

FCP-connected SCSI

Processors equipped with FICON channels interface with SCSI drives through the Fibre Channel Protocol (FCP). Full support for FCP is provided by z/VM and z/VSE, while z/OS offers partial compatibility via IOSFBA.

Access

The programming interface macros and routines used for device interaction are collectively known as access methods, commonly ending with Access Method.

DOS/360 and its successors

From DOS/360 through z/VSE, DASD datasets are accessed using:

Logical IOCS (LIOCS)
- sequential access method (SAM)
- direct access method (DAM)
- indexed sequential access method (ISAM)
- virtual storage access method (VSAM)
Physical IOCS (PIOCS)
- Execute channel program (EXCP)

OS/360 and its successors

DASD datasets under OS/360 through z/OS are supported by:

Basic sequential access method (BSAM)
Basic indexed sequential access method (BISAM)
Queued sequential access method (QSAM)
Queued indexed sequential access method (QISAM)
Basic partitioned access method (BPAM)
Basic direct access method (BDAM)
Virtual storage access method (VSAM)
Execute Channel Program (EXCP)
Execute Channel Program in Real Storage (EXCPVR)

Beginning with OS/VS2 Release 2 and continuing through z/OS, all access methods, including EXCP and EXCPVR, utilize the privileged STARTIO macro.

Terminology

In IBM's 1964 publication "IBM System/360 System Summary," the term File was used to describe what are now known as DASD devices, characterized as "random access storage." Concurrent IBM documentation referred to these devices as "direct access storage devices" without employing an acronym.

The acronym DASD was first introduced publicly in IBM's March 1966 "Data File Handbook." Its usage outside IBM began around 1968 and expanded until approximately 1990, after which it gradually declined.

As drum and data cell technologies became obsolete, DASD evolved to broadly encompass disk, flash, and optical storage devices. Present-day mainframe DASD seldom consists of individual disk drives; instead, it generally refers to extensive disk arrays configured with RAID. Modern hardware frequently emulates CKD on FBA devices.

References