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Adirect-access storage device (DASD) (pronounced/ˈdæzdiː/) is asecondary storage device in which "each physical record has a discrete location and a unique address". The term was coined byIBM to describe devices that allowedrandom access to data, the main examples beingdrum memory andhard disk drives.[1] Later,optical disc drives andflash memory units are also classified as DASD.[2][3]
The term DASD contrasts withsequential access storage device such as amagnetic tape drive, andunit record equipment such as apunched 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, anddirect.
The DASD storage class includes both fixed and removable media.
IBM mainframes access I/O devices including DASD throughchannels, a type of subordinate mini-processor.Channel programs write to, read from, and control the given device.[4] IBM direct access storage devices prior toSystem/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 toIBM 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, orMBBCCHHR[5] Channel programs address DASD using a six byte seek address[6] and a five byte record identifier[7] (CCHHR).
For devices prior to extended address volumes, the seek address is
| Device | Byte | |||||
|---|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | 4 | 5 | |
| Disk | 0 | 0 | Cylinder | Head | ||
| Drum | 0 | 0 | 0 | Cylinder | 0 | Head |
| Data cell | 0 | cell | subcell | strip | Cylinder | Head |
When the 2321 data cell was discontinued in January 1975,[8] 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 aslogical records, and the format on DASD[a] asblocks orphysical records. One block might contain several logical (or user) records or, in some schemes, calledspanned 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 asQSAM, are responsible forblocking anddeblocking logical records as they are written to or read from external media. The basic-access methods, such asBSAM, require the user program to do it.
CKD is an acronym forCount Key Data, the physical layout of a block on a DASD device, and should not be confused with BBCCH and CCHHR, which are the addresses used by the channel program. CTR in this context may refer to either type of address, depending on the channel command.[citation needed]
In 1979 IBM introducedFixed-block architecture (FBA) 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. As part of the FBA interface IBM introduced new channel commands for asynchronous operation that are very similar to those introduced for ECKD.
For some applications, FBA not only offers simplicity, but an increase in throughput.
FBA is supported byVM/370 andDOS/VSE, but notMVS[b] or successor operating systems in the OS/360 line.
Processors withFICON channels can access SCSI drives usingFibre Channel Protocol (FCP). While z/VM and z/VSE fully support FCP, z/OS provides only limited support through IOSFBA.
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Some programming interface macros and routines are collectively referred to asaccess methods with names ending inAccessMethod.
DOS/360 throughz/VSE support datasets on DASD with the following access methods:[citation needed]
OS/360 throughz/OS support datasets on DASD with the following access methods:[citation needed]
InMVS, starting withOS/VS2 Release 2 and continuing throughz/OS, all of the access methods including EXCP[VR], use the privilegedSTARTIO macro.
IBM in its 1964 first version of the "IBM System/360 System Summary" used the termFile to collectively described devices now called DASD. Files provided "random-access storage'"[4] At the same time IBM's product reference manual described such devices as "direct-access storage devices[9]" without any acronym.
An early public use of the acronym DASD is in IBM's March 1966 manual, "Data File Handbook.[10]" The earliest non-IBM use of the acronym DASD found by the "Google ngram viewer" to refer to storage devices dates from 1968.[11] From then on use of the term grew exponentially until 1990 after which its usage declined substantially.[12]
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 largedisk arrays utilizingRAID schemes. Current devices emulate CKD on FBA hardware.
The IBM 2841 Storage Control Unit provides for the attachment of direct-access storage devices to the IBM System/360.