[1] 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, but typically provide a greater level of functionality.
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.
This is usually front ended by the Execute Channel Program (EXCP) macro for application programmer convenience.
[2] Access methods provide: Unlike systems derived from Unix, where all files and devices are considered to be an unformatted stream of bytes, mainframes offer a variety of data options and formats, such as varying types and sizes of records, and different ways of accessing data, such as via record keys.
Basic access methods read or write one physical record – block – at a time.