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:
“Write to the point: say immediately what you want to say most, even if it doesn’t “come first.” There are three reasons for doing this. First, you will then have said it, even if nothing else gets said. Second, your readers will then have read it, even if they read no more. Third, having said it, you are likely to have to say something more, because you will have to explain and justify what you chose to say.”
—Bill Stott (b. 1940)
“There is no way of conveying to the corpse the reasons you have made him one—you have the corpse, and you are, thereafter, at the mercy of a fact which missed the truth, which means that the corpse has you.”
—James Baldwin (1924–1987)
“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)
“Oh, the holiness of always being the injured party. The historically oppressed can find not only sanctity but safety in the state of victimization. When access to a better life has been denied often enough, and successfully enough, one can use the rejection as an excuse to cease all efforts. After all, one reckons, “they” don’t want me, “they” accept their own mediocrity and refuse my best, “they” don’t deserve me.”
—Maya Angelou (b. 1928)
“Crime is terribly revealing. Try and vary your methods as you will, your tastes, your habits, your attitude of mind, and your soul is revealed by your actions.”
—Agatha Christie (1891–1976)