It 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 API for programmers to transfer data to or from device, and could be compared to device drivers in modern operating systems.
Without access methods, a programmer must write 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.
This processor requires programs written with special instructions, called Channel Command Words (CCWs). Programming those is a complex and arduous task
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