Object access method

Object access method (OAM) is an access method under z/OS which is designed for the storage of large numbers of large files, such as images. It has a number of distinguishing features, e.g. compared to VSAM:

OAM is used in conjunction with DB2. An example use case for OAM would be storing medical images in a DB2 database running under z/OS.

History

OAM was created in the 1980s "as a prototype product for an insurance company to replace microfiche." Initially OAM supported optical storage and magnetic disks. In the 1990s support for magnetic tape was added. In 2011 support was added for storage of objects in a z/OS unix file system—either zFS or NFS.

Access method

An access method is a function of a mainframe operating system that enables access to data on disk, tape or other external devices. They were introduced in 1963 in IBM OS/360 operating system. Access methods provide an application programming interface (API) for programmers to transfer data to or from device, and could be compared to device drivers in non-mainframe operating systems.

Purpose of access methods

System/360 and successor systems perform input/output using a special program for an I/O channel, a processor dedicated to control peripheral storage device access and data transfer to and from main memory. Channel programs are composed of channel command words (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a START IO instruction issued by the operating system. This is usually front ended by the Execute Channel Program (EXCP) macro for application programmer convenience. EXCP issues an SVC (supervisor call instruction) that directs the operating system to issue the START IO on the application's behalf.