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. These channel programs are composed of special instructions, called channel command words (CCWs). Programming those is a complex task requiring detailed knowledge of the hardware characteristics. Channel programs are initiated by a STARTIO macro 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 directs 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.
- Ease 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 and system security.
Read more about this topic: Access Method
Famous quotes containing the words reasons for, reasons, introducing, access and/or methods:
“With no matter what human being, taken individually, I always find reasons for concluding that sorrow and misfortune do not suit him; either because he seems too mediocre for anything so great, or, on the contrary, too precious to be destroyed.”
—Simone Weil (1909–1943)
“One of the reasons for the failure of feminism to dislodge deeply held perceptions of male and female behaviour was its insistence that women were victims, and men powerful patriarchs, which made a travesty of ordinary people’s experience of the mutual interdependence of men and women.”
—Rosalind Coward (b. 1953)
“The beginner’s well-known propensity for obtruding upon his own privacy, by introducing himself, or a vicar, into his first novel, owes less to the attraction of a ready theme than to the relief of getting rid of oneself, before going on to better things.”
—Vladimir Nabokov (1899–1977)
“The last publicized center of American writing was Manhattan. Its writers became known as the New York Intellectuals. With important connections to publishing, and universities, with access to the major book reviews, they were able to pose as the vanguard of American culture when they were so obsessed with the two Joes—McCarthy and Stalin—that they were to produce only two artists, Saul Bellow and Philip Roth, who left town.”
—Ishmael Reed (b. 1938)
“It would be some advantage to live a primitive and frontier life, though in the midst of an outward civilization, if only to learn what are the gross necessaries of life and what methods have been taken to obtain them.”
—Henry David Thoreau (1817–1862)