Usage edit
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cleanup: This is not an article! All the article-like stuff is to be moved, i.e. to the Access method article: All the intro paragraphs as well as all the latter chapters.
The OS/360 and successors I/O access methods are programming APIs on the OS/360 mainframe computer operating system and its successors, such as OS/VS1, OS/VS2 (SVS), MVS, OS/390, and z/OS, used by programmers to implement input/output operations on data sets (files) and telecommunications networks. They are by convention referenced to with ...AM acronyms, standing for ... Access Method.
Including EXCP, EXCPVR and STARTIO in a separate class edit
These are general purpose, low-level I/O device methods of access, and not "access methods" in the conventional use of this term.
In pre-MVS instances of the OS, EXCP was the lowest level method of access, and all access methods utilized EXCP.
In SVS, EXCPVR was added to enable programs which were executing in real storage to avoid much of the overhead of EXCP. This was continued in MVS.
In MVS and later instances of the OS, STARTIO replaced both forms of EXCP as the lowest level method of access. The so-called virtual access methods utilize STARTIO directly, or they have their own SVC which, in turn, utilizes STARTIO. The legacy access methods also utilize STARTIO, but go through EXCP to do so.
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