data set organization
DSORG
(DSORG) An IBM term for file structure. These include PS physical sequential, DA direct access, IS indexed sequential, PO partitioned (a library). This system dates from OS/360, and breaks down beginning with VSAM and VTAM, where it is no longer applied.
Sequential and indexed data sets can be accessed using either a "basic" or a "queued" "access method." For example a DSORG=PS file can use either BSAM (basic sequential access method) or QSAM (queued sequential access method). It can also be processed as a direct file using BDAM. Likewise a library can be processed using BPAM (basic partitioned access method), BSAM, QSAM, or BDAM. DSORG and access method are somewhat, but not completely, orthogonal.
The "basic" access method deals with physical blocks rather than records, and usually provides more control over the specific device. Each I/O operation using the "basic" access method reads or writes a single block. A "basic" read or write starts an asynchronous I/O operation, and the programmer is responsible for waiting for completion and checking for errors.
The "queued" access method deals with logical records and provides blocking and deblocking services. It is "queued" because it provides read-ahead and write-behind services. While a program is processing records in one input block, for example, QSAM may be reading one or more blocks ahead. Queued "get" or "put" operations are synchronous as far as the programmer is concerned. The operation is complete when the next logical record has been successfully processed.
EXCP (Execute Channel Program) is a lower-level method of accessing data.
IBM manuals usually named "Data Administration Guide", e.g. SC26-4505-1 for MVS/ESA DFP 3.1, provide more detail about data set organizations and access methods.