Editing Direct-access storage device (section)

==Architecture==
IBM mainframes access I/O devices including DASD through ''[[Channel I/O|channels]]'', a type of subordinate mini-processor. ''[[Channel program]]s'' write to, read from, and control the given device.<ref name="360Sum" />

===CTR (CHR)===
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 ('''BBCCHH''') and a five byte record  identifier ('''CCHHR'''). {{Citation needed|date=September 2024}}
*'''M''' represents the extent number within the allocation
*'''BB''' representing the '''B'''in (from [[IBM 2321 Data Cell|2321 data cells]]), 
*'''CC''' representing the '''C'''ylinder, 
*'''HH''' representing the '''H'''ead (or track), and 
*'''R''' representing the '''R'''ecord (block) number. 
When the 2321 data cell was discontinued in January 1975,<ref>{{cite web
|url         = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_2321.html
|archive-url = https://web.archive.org/web/20060226172127/http://www-03.ibm.com/ibm/history/exhibits/storage/storage_2321.html
|url-status  = dead
|archive-date = February 26, 2006
|title       = IBM Archives: IBM 2321 data cell drive
|author      = IBM Corporation
|date = 23 January 2003
|access-date  = 8 Nov 2011
}}</ref> 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{{efn|The concept of physical record also applies to other device types, e.g., tape.}} as [[Block (data storage)|blocks]] or ''physical records''. One block might contain several logical (or user) 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 [[Basic_sequential_access_method|BSAM]], require the user program to do it. 

====CKD====
{{main|Count key data}}
CKD is an acronym for [[Count 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|date=September 2024}}

===FBA===
In 1979 IBM introduced [[fixed-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 by [[VM (operating system)|VM/370]] and [[VSE (operating system)|DOS/VSE]], but not [[MVS]]{{efn|However, the privileged routine IOSFBA provides limited access to these devices.}} or successor operating systems in the OS/360 line.

===FCP attached SCSI===
Processors with [[FICON]] channels can access SCSI drives using [[Fibre Channel Protocol]] (FCP). While z/VM and z/VSE fully support FCP, z/OS provides only limited support through IOSFBA.