There is only one place in z Architecture Principles of Operation where the word substantially is used. Even more surprising than its use is the fact that it appears immediately before the word "accurate". My interest in the reference bit, though passing, is sincere since I would like to hang my hat on the validity of that indicator. Substantially means " to a great extent or degree". In system z architecture, I am unfamiliar with this level of accuracy.
For your reference (no pun intended) the paragraph in z POPS is: "The record provided by the reference bit is substantially accurate. The reference bit may be set to one by fetching data or instructions that are neither designated nor used by the program, and, under certain conditions, a reference may be made without the reference bit being set to one. Under certain unusual circumstances, a reference bit may be set to zero by other than explicit program action." Although Seinfeld had to have written the previous 67 words, it is possible (age 21 at the time) for him to have contributed the original paragraph from the 1975 POPS and simply updated the paragraph for System z. The 1975 paragraph follows. "The record of references provided by the reference bit is substantially accurate. The reference bit may be turned on by fetching data or instructions that are neither designated nor used by the program, and, under certain conditions, a reference may be made without the reference bit being turned on. Under certain unusual conditions, a reference bit that is on may be turned off by other than explicit program action. "Turned on" and "Turned off" apparently gave way to the more precise "set to one" and "set to zero" for those so technically challenged that on/off had lost all meaning. Think about it; the technical writers were striving for more precision in their description than the "thing" being described actually provided. Does anyone have a clue what sentences 2 and 3 (substantially) mean in either paragraph and would anyone venture a guess as to why, after 40 years of architectural excellence, the reference bit can't be more than "substantially accurate"? My curiosity is killing me. --. .- .-. -.-- Gary Dennis
