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)
“One who is publicly honest about himself ends up by priding himself somewhat on this honesty: for he knows only too well why he is honest—for the same reasons another person prefers illusion and dissimulation.”
—Friedrich Nietzsche (1844–1900)
“The man who thrusts his manners upon me does as if he were to insist on introducing me to his cabinet of curiosities, when I wished to see himself.”
—Henry David Thoreau (1817–1862)
“The professional celebrity, male and female, is the crowning result of the star system of a society that makes a fetish of competition. In America, this system is carried to the point where a man who can knock a small white ball into a series of holes in the ground with more efficiency than anyone else thereby gains social access to the President of the United States.”
—C. Wright Mills (1916–1962)
“We are lonesome animals. We spend all our life trying to be less lonesome. One of our ancient methods is to tell a story begging the listener to say—and to feel—”Yes, that’s the way it is, or at least that’s the way I feel it. You’re not as alone as you thought.””
—John Steinbeck (1902–1968)