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)
“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)
“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)
“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)