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:
“One of the many reasons for the bewildering and tragic character of human existence is the fact that social organization is at once necessary and fatal. Men are forever creating such organizations for their own convenience and forever finding themselves the victims of their home-made monsters.”
—Aldous Huxley (1894–1963)
“Here we also see: what this divinity lacks is not only a sense of shame—and there are also other reasons for conjecturing that in several respects all of the gods could learn from us humans. We humans are—more humane.”
—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 Hacker Ethic: Access to computers—and anything which might teach you something about the way the world works—should be unlimited and total.
Always yield to the Hands-On Imperative!
All information should be free.
Mistrust authority—promote decentralization.
Hackers should be judged by their hacking, not bogus criteria such as degrees, age, race, or position.
You can create art and beauty on a computer.
Computers can change your life for the better.”
—Steven Levy, U.S. writer. Hackers, ch. 2, “The Hacker Ethic,” pp. 27-33, Anchor Press, Doubleday (1984)
“There are souls that are incurable and lost to the rest of society. Deprive them of one means of folly, they will invent ten thousand others. They will create subtler, wilder methods, methods that are absolutely DESPERATE. Nature herself is fundamentally antisocial, it is only by a usurpation of powers that the organized body of society opposes the natural inclination of humanity.”
—Antonin Artaud (1896–1948)