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:
“Could truth perhaps be a woman who has reasons for not permitting her reasons to be seen? Could her name perhaps be—to speak Greek—Baubo?... Oh, those Greeks! They understood how to live: to do that it is necessary to stop bravely at the surface, the fold, the skin, to adore the appearance, to believe in forms, in tones, in words, in the whole Olympus of appearance! Those Greeks were superficial—out of profundity!”
—Friedrich Nietzsche (1844–1900)
“From things that have happened and from things as they exist and from all things that you know and all those you cannot know, you make something through your invention that is not a representation but a whole new thing truer than anything true and alive, and you make it alive, and if you make it well enough, you give it immortality. That is why you write and for no other reason that you know of. But what about all the reasons that no one knows?”
—Ernest Hemingway (1899–1961)
“The natural historian is not a fisherman who prays for cloudy days and good luck merely; but as fishing has been styled “a contemplative man’s recreation,” introducing him profitably to woods and water, so the fruit of the naturalist’s observations is not in new genera or species, but in new contemplations still, and science is only a more contemplative man’s recreation.”
—Henry David Thoreau (1817–1862)
“Power, in Case’s world, meant corporate power. The zaibatsus, the multinationals ..., had ... attained a kind of immortality. You couldn’t kill a zaibatsu by assassinating a dozen key executives; there were others waiting to step up the ladder; assume the vacated position, access the vast banks of corporate memory.”
—William Gibson (b. 1948)
“The ancient bitter opposition to improved methods [of production] on the ancient theory that it more than temporarily deprives men of employment ... has no place in the gospel of American progress.”
—Herbert Hoover (1874–1964)