Basic direct access method
Basic Direct Access Method, or BDAM is an
, and related high-end operating systems.Description
Basic, in IBM terminology, indicates a lower-level access method that deals with data sets in terms of physical blocks and does not provide either read-ahead, called anticipatory buffering, or synchronization — that is, the user program has to explicitly wait for completion of each input/output event.[2]: p.31
With BDAM, "the programmer can directly store and retrieve a block by specifying either its actual device address, its relative position within a data set (relative block number), or the relative track within a data set at which the system is to begin a search.[2]: p.63
BDAM provides no index or structure to the file except as programmed by the application. In many applications, a hashing or randomizing function may be used to assign the block address based on a key in the data. If physical
The BDAM application program interface can be compared with the interface offered by open, read, write and close calls (using file handles) in other operating systems such as Unix and Windows.
BDAM is still supported by IBM as of 2012. Because of its dependence on physical device geometry new IBM direct-access devices used with z/OS emulate
Application program interface
The programmer specifies DSORG=DA in his
SPACE=(,,CONTIG) on the DD statement.
Direct data sets must be preformatted before use by opening them as output and writing all the blocks sequentially. This can load all "dummy" records or load initial data.[3]
As a basic access method BDAM reads and writes member data in
CHECK macro has to be used to wait for completion of a specific operation before the data can be accessed or the data bufferRecords in a direct data set can contain user-specified recorded keys of up to 255 bytes—all keys in a file must be the same length. Reads and writes can specify a key in addition to a disk address, and BDAM will search starting at the requested block up to the entire area of the file for a record with a matching key.[2] This allows multiple keys hashing to the same track to be handled automatically by the I/O subsystem.
Disk addresses
BDAM accepts disk addresses as either actual device addresses, as relative track addresses, or as relative block numbers.
Device addresses are eight byte fields in the form MBBCCHHR. All subfields are binary numbers.[5]
- M is the extent number for files with more than one extent (non-contiguous files).
- BB is zero. Previously this indicated the bin on an IBM 2321 Data Cell.
- CC is the absolute cylinder number on the device.
- HH is the track (head) number.
- R is the record number on the track. Record zero contains control information for the track and can not be used.
Specifying device addresses makes the dataset unmovable.[2]: p.106
Relative track addresses are three byte fields in the form TTR. All subfields are binary numbers.
- TT is the relative track number in the file, starting with zero.
- R is the record number on the track.
Relative block numbers are four byte (fullword) binary numbers indicating the block number in the file. The first block is block 0. This form of addressing can only be used with fixed length blocks (RECFM=F).[6]: p.106
See also
- Count Key Data
References
- ^ IBM System/360 Operating System Basic Direct Access Method Program Logic Manual (PDF). IBM. July 1967. Y28-6617-3.
- ^ a b c d IBM System/360 Operating System Data Management (PDF). IBM. April 1966. C28-6537-1.
- ^ CICS and BDAM, IBM infocenter
- ^ IBM Corporation (June 1973). OS Data Management Macro Instructions (PDF). Retrieved August 19, 2016.
- ^ IBM Corporation. "Executing Direct Access Programs". IBM.
- ^ OS Data Management Services Guide (PDF). IBM. July 1973. GC26-3746-2.
External links
- Introduction to IBM Direct-Access Storage Devices and Organization Methods (PDF). IBM. February 1974. GC20-1649-9. Archived from the original (PDF) on 2011-03-22. Retrieved 2012-09-24.