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:
“While there are practical and sometimes moral reasons for the decomposition of the family, it coincides neither with what most people in society say they desire nor, especially in the case of children, with their best interests.”
—Robert Neelly Bellah (20th century)
“From things that have happened and from things as they exist and from all things that you know and all those you cannot know, you make something through your invention that is not a representation but a whole new thing truer than anything true and alive, and you make it alive, and if you make it well enough, you give it immortality. That is why you write and for no other reason that you know of. But what about all the reasons that no one knows?”
—Ernest Hemingway (1899–1961)
“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)
“Whilst the rights of all as persons are equal, in virtue of their access to reason, their rights in property are very unequal. One man owns his clothes, and another owns a country.”
—Ralph Waldo Emerson (1803–1882)
“I conceive that the leading characteristic of the nineteenth century has been the rapid growth of the scientific spirit, the consequent application of scientific methods of investigation to all the problems with which the human mind is occupied, and the correlative rejection of traditional beliefs which have proved their incompetence to bear such investigation.”
—Thomas Henry Huxley (1825–95)