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:
“Metaphysics is the finding of bad reasons for what we believe upon instinct; but to find these reasons is no less an instinct.”
—F.H. (Francis Herbert)
“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 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)
“Lesbian existence comprises both the breaking of a taboo and the rejection of a compulsory way of life. It is also a direct or indirect attack on the male right of access to women.”
—Adrienne Rich (b. 1929)
“If men got pregnant, there would be safe, reliable methods of birth control. They’d be inexpensive, too.”
—Anna Quindlen (b. 1952)