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:
“The more one analyses people, the more all reasons for analysis disappear. Sooner or later one comes to that dreadful universal thing called human nature.”
—Oscar Wilde (1854–1900)
“As if her velvet helmet high
Did turret rationality.
She fans her wing up to the winde
As if her Pettycoate were lin’de
With reasons fleece, and hoises saile
And humming flies in thankfull gaile”
—Edward Taylor (1645–1729)
“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)
“Power, in Case’s world, meant corporate power. The zaibatsus, the multinationals ..., had ... attained a kind of immortality. You couldn’t kill a zaibatsu by assassinating a dozen key executives; there were others waiting to step up the ladder; assume the vacated position, access the vast banks of corporate memory.”
—William Gibson (b. 1948)
“I believe in women; and in their right to their own best possibilities in every department of life. I believe that the methods of dress practiced among women are a marked hindrance to the realization of these possibilities, and should be scorned or persuaded out of society.”
—Elizabeth Stuart Phelps (1844–1911)