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 API for programmers to transfer data to or from device, and could be compared to device drivers in non-mainframe operating systems.

Reasons for introducing access methods

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. Channel programs are kicked off from mainframe programs by a "startIO" instruction issued by the operating system - this is usually front ended by the Execute Channel Program (EXCP) macro for application programmer convenience. This macro issues an SVC (supervisor call instruction) that asks the operating system to issue the startIO on the application's behalf.

Access methods provide:

• Ease of programming - programmer would no longer deal with a specific device procedures, including error detection and recovery tactics in each and every program. A program designed to process a sequence of 80-character records would work no matter where the data are stored.
• Ease of hardware replacement - programmer would no longer alter a program when data should be migrated to newer model of storage device, provided it supports the same access methods.
• Easy shared data set access - an access method is a trusted program, that allows multiple programs to access the same file, while ensuring the basic data integrity.